Given its pricing, the BMW definitely needs to be good. At £92,475 it’s almost £24k beyond the base M2 and £3k more than even the M3 Competition xDrive. Add this test car’s £8800 carbon-ceramic brakes and it breezes past six figures. Production numbers are officially limited, which may improve the chances of it making financial sense, though exactly how limited we do not know; nor do we know if this CS will eventually be topped by a CSL.

The new BMW is here to meet the soon-to-retire Porsche 718 Cayman 4.0 GTS, a car now five years old and not that far removed from the 981-generation Cayman we first drove in 2012. No matter, the 4.0 GTS remains a dynamic benchmark.

Why these two cars? Sharper and more exciting than regular BMW M or Porsche models, yet not so extreme (or expensive) as CSL or GT specials, BMW’s CS and Porsche’s GTS variants have become the default ‘sweet spots’ of their line-ups.

James Dennison’s arrival in the Cayman announces itself with a howling flat-six exhaust note and spitty PDK gearshifts as he climbs from knots of damp, tighter turns below to the open expanses, clear sight lines and startlingly good conditions we’re now basking in. Seeing the BMW and Porsche parked side by side in piercing autumn sunshine is a stark reminder that these German rivals continue to do the entry-level rear-drive sports coupe really quite differently.

The Cayman’s voluptuous styling wraps round a two-seat cabin and a 4.0-litre naturally aspirated flat-six mounted low and amidships. It’s 265kg lighter than the 1700kg BMW, as well as being compact and lower slung, with lines that succinctly communicate the fingertippy delicacy of this exceptional driving experience.

Positively old-school peak power of 395bhp and weight of just 1435kg contrasts with the Porsche’s thoroughly modern price of £75,300, albeit that’s £28k shy of the cheapest 911 and £17k short of the M2 CS. GTS trim adds 20mm lower sports suspension with adaptive dampers, Porsche Torque Vectoring with a mechanical locking diff, and the Sport Chrono pack, complete with mode switch on the steering wheel, dynamic transmission mounts and the little stopwatch on the dash.

To that, this car adds a further £13k in optional equipment. Nothing materially affects the driving experience, with most outlay going on £1824 Carmine Red paint, £2042 18-way sports seats and another – somewhat astonishing – £5k or so on cosmetic, comfort and audio upgrades for an interior that’s nicely assembled and laid out, if lacking much in the way of surprise and delight.

A word of warning on those optional 18-way chairs. My 6ft 1in frame normally slots into a Cayman so snugly it’s as if I’ve been wheeled down the Zuffenhausen production line and the car built around me. Perversely, given all their infinite adjustment, these pricey perches don’t motor quite far enough back and feel a little higher than in other Caymans. The M2 driver can drop down in a more satisfying manner.

Techies will also find the Porsche’s (perfectly serviceable) infotainment system lacking in pizzazz, but you can’t argue with the Cayman’s focus – an otherwise perfect driving position, compact proportions that the 911 grew out of two generations ago, and a nice small-diameter steering wheel with a thin rim that makes the BMW’s helm feel over-inflated.

You also get physical buttons in the Porsche. They’re the hot new thing for some cars but they feature here simply because – like a stopped clock – the Cayman has hung in there and the 2010s have come back into fashion.

By comparison, stepping inside the BMW feels like someone’s mobilised my living room. There are big screens and space for the family, if no central holder for a drinks bottle because the CS carbonfibre centre console deletes it, which is both bizarre and frustrating. Nonetheless, a roomy, high-tech cabin is a key weapon in the G87-generation M2’s armoury – it’s just so usable and feels so much more up-to-date than the rather classic Cayman.

JD worries the M2’s become too big, noting it’s the same width and barely a passenger lighter than an M4 CS. ‘The BMW’s usability is really excellent,’ he concedes. ‘The cabin is anything but stripped-out despite the extra performance focus and has everything you need for everyday driving – it’s just that, for me, half the appeal of a sports car on UK roads is less width and weight.’

A bluff profile, Lego-like wheelarches and a bonnet creased like its 3.0-litre turbo six is trying to punch its way out all carry over from the standard M2. Visual CS differentiation instead comes via more open vents that leave the cooling apparatus looking vulnerable, and a full carbonfibre bootlid with a flourish that’s more skate ramp than duck tail. Finished in Sapphire Black metallic with bespoke gold wheels, this CS is a screaming chicken and CB aerial away from a Smokey and the Bandit tribute, which I’m all here for.

The M2 CS comes with carbon-shelled bucket seat that are optional on the regular M2. Bolsters like blades require caution as you climb aboard, and the rollercoaster-style carbon bit between your legs continues to feel a bit odd, but they’re pleasingly snug and surprisingly comfortable once you’re ensconced, even for long schleps. They’re electrically adjusted and heated too.

Thumb the starter, wake the engine with its deep growl (which sounds a little raspier than standard, certainly more crackly on the over-run) and it takes very little time to detect some pretty marked differences to the stock M2 – a car I liked from the off but which leaves room for a less isolated, faster and somewhat sharper sibling.

The ride is tetchy at low speeds, and more feeling tingles up from Michelin Pilot Sport 4S rubber to excite that chunky alcantara rim. Throttle response is also perhaps a little over-egged in my preferred Sport mode – all edgy on tip-in before settling down into the meat of its travel – and the eight-speed auto gets a slightly clumsy kick in its third and most aggressive mode. There is no manual transmission.

But after that patchy prelude, I start to bond with the M2 on some extremely familiar B-roads just a few miles from home. Above 30mph that initially lumpy ride becomes smoother, the eager throttle mapping becomes second nature, and when I knock back the shifts to their mid setting, the powertrain eases into buttery bliss. Hold a gear to overtake and the M2 surges forwards effortlessly, if still with a large helping of drama. You can’t half make progress in this thing.

Configured in Sport mode for everything, except Comfort for the dampers, the CS settles into a fluid, controlled stride, its six-cylinder engine a slightly dark, omnipotent force churning away up ahead and the chassis melding elasticity and stability with some very welcome extra feel. Experimentation with Sport damping reveals the second of three adaptive settings is the best for relatively smooth roads and brings benefits in tightened body control, but more limber-limbed Comfort is almost always preferable.

For such a pronounced difference in feel over the stock M2, hardware changes are actually pretty minimal. They include 8mm lower springs, stiffer engine mounts, forged alloys (275/35 ZR19 front and 285/30 ZR20 rear tyres are unchanged from the base model), while the Driver’s pack is thrown in too, which uncorks top speed to 188mph. Unlike any other modern CS BMW, this M2 is also purely rear-wheel drive.

For the rest of its transformation, the CS relies on calibration magic – for its throttle map, M3-matching power bump, adaptive dampers, e-diff, stability control and electric power steering. It’s the sort of stuff that risks being imperceptible to anyone but chassis engineers, but it all manifests as one holistic and highly tangible shift in character.

Scudding over the Moors, the balance is definitely more oversteer biased than a regular M2. On colder tyres and stability control given its longer M Dynamic mode leash, the CS pivots into turns with surprisingly little inertia – the steering feels more eager off the straight-ahead to me, not communicative enough initially for JD. And it lets you wind on a decent amount of corrective lock before intervening under power.

At first the balance is agility bordering on instability, in a manner that recalls the last-generation M4 GTS – especially how swiftly the rear snaps to attention to follow the steering input, with the usual overshoot as the rear axle gathers itself up all but eliminated.

Work heat into the tyres, though, and grip levels step up to match the flighty response. The CS shrugs off entry speeds that’d have most passengers pulling the eject cord and refuses to let go at the rear even when the tyres are on tippy toes and the fronts are fully loaded. Instead you get to dictate the cornering line with the throttle, something that could otherwise be a huge leap of faith but is de-risked here by excellent 10-stage traction control.

When fully unshackled the M2 CS slides more progressively than the base car. Perhaps the extra oomph helps overwhelm grip more decisively, but gut feel says diff tuning plays a significant role – I’d guess more open off-throttle as you turn, more aggressively locked as you feed in power, but whatever the special sauce, there’s definitely a big shift in the M2’s cornering balance.

Performance is another area where you frown at the spec sheet and wonder just how much of the 49bhp uplift and 30kg weight saving you can feel, but the fact that the 0-124mph benchmark is done in 11.7sec (1.2sec faster) gives some indication of the grunt being served here.

First and second gears blow by in a blur and are arguably too exaggerated, but by the time I settle into the more relaxed third and fourth cogs the CS is just stretching out on one indulgent surge of power through to 6500rpm (although the powerband actually feels wider than that suggests).

There is a little lag until 2000rpm or so, and peak torque of 479lb ft from not much after is excessive in the wet for a purely rear-drive car (just keep the traction systems on) but in the dry it’s sublime, allowing you to really dig into this chassis and exploit its adjustability. It’s more agile and with more attitude than the also-excellent M4 CS, and clearly differentiated from its base sibling. I like it a lot.

It’s a daunting bar for the Cayman to vault, but the ageing Porsche still takes the fight convincingly to the BMW – and thankfully looks set to return for a new generation, Porsche having rowed back on making the next-gen Boxster and Cayman purely electric. Toppy models will continue to be petrol powered, says boss Oliver Blume, which we hope will include the GTS.

It’s like the genesis of the 4.0 GTS all over again, when Porsche introduced the four-pot Cayman and Boxster, then rustled up the naturally aspirated GTS flat-six when purists lost their minds. Born from pragmatism (and the GT4 parts bin), it helped transform the GTS from what was effectively a tempting options bundle into a much more clearly defined car – no longer a rich man’s S, it became the next best thing to a GT4 while also being easier to live with.

Almost six years on, the GTS remains an incredibly special experience, defined by the purity and cohesion of its drive, all centred around a remarkable engine. All mechanical grit at idle, warm bass in the meat of its powerband and howling treble at higher rpm, it’s so tuneful I want to blow down its intake plenum and play its manifolds like a six-into-one wind instrument.

Response is always on the button, but crank up the little dial on the steering wheel and both throttle and gearshifts become even more vital. It’s a synaptic, highly mechanical connection, even with the brilliant PDK transmission.

‘It’s a shame they don’t offer it with a manual gearbox,’ says Dennison, ‘but the Cayman leads for throttle response, gearbox and soundtrack – the CS engine is strong and exciting, but the noise won’t win any awards and it can’t match the Porsche’s response.’

If you’re stepping out of the BMW and into the Porsche, you initially experience the weird illusion that it’s down a couple of cylinders. Long gearing, particularly in first and second, combines with significantly lower peak torque of 317lb ft at 5000rpm to really knock the wind from its sails.

But this motor thrives on revs, rewarding the commitment you put in to summit at 7000rpm. By the time you reach third gear the speed has overcome inertia to such an extent that the ratio feels perfectly well suited. Impeccable dynamics then do the rest, ensuring momentum stays high simply because you can flow the Cayman so beautifully through bends. Get into a groove and you can go at the Moors in the GTS like a dog digging up a back yard for a bone.

It’s saying something that the M2’s turn-in response actually makes the Porsche helm seem a little sleepy, but the overall feel is more fluid and natural than its BMW rival. It’s in the Porsche’s supple damping, the perfect weight and consistency of its more linear and organic steering, and weight distribution that simply feels inherently better – feathery at the front, planted at the rear, and with an adjustability that quickly builds a dialogue between car and driver.

Through one long, clearly sighted sweeper over moorland, I just point the steering, pin the throttle and the Cayman glides over the buckled surface like a rolling pin over dough. Even really gnarly mid-corner bumps fail to ruffle its flow. When I swap back into the M2, I’m much more aware of its suspension fighting to control the considerable extra mass through the same corner, and that it becomes more ragged and less satisfying as a result.

Lower performance also means the Cayman can get away with a small footprint for its P Zero rubber, so while the 20-inch diameter is big, the 235- front and 265-section rears are pretty narrow by modern performance-car standards. This all means you can use much more of the Porsche’s potential on the road, playing with its balance by tweaking steering and lifting the throttle mid-bend, where the M2’s interactivity is accessed with a heavier right foot as you fire out of the corner (in addition to steering bungs and throttle lifts). This is why JD describes the Porsche as the more intuitive car to exploit, and one that he finds better suited to UK roads.

The Porsche’s performance also translates better to wet conditions, and relies far less on its traction- and stability-control systems to keep the average driver out of trouble than the BMW. This is PG-rated handling fun to the M2’s adult entertainment.

The Cayman GTS and M2 CS represent such different engineering philosophies – the BMW effectively a downsized, hotted-up mainstream saloon, the Porsche descended from a blue-blood sports car – yet they’ve converged on more or less the same ground with very impressive results, albeit with significant differences.

As the sun starts to dip and I try to decide which is the better car, I keep back-to-backing the Porsche and BMW down the same stretch of road and failing to return with a firm answer – marvelling at the Cayman’s sublime balance one minute, then falling for the M2 CS’s extra performance and drama the next.

So this will be a verdict that’s ridiculously difficult to call…