Showroom Appeal:

Porsche's mentality is less for more. The shedding of 55kg from the Cayman S comes courtesy of aluminium door skins (as found on 911 GT3), fixed carbon fibre bucket seats, removal of the stereo and air conditioning (though both can be retrofitted), shedding 10 litres from the fuel tank and adding the 19-inch light rims from the Boxster Spyder. From the sound of it, the engine cover is significantly thinner too, plus there are fabric door handles and no cowl over the instruments. Few off-the-shelf cars look this focused on speed.

The BMW, on the other hand, doesn't so much refine its 1 Series basis as ditch it entirely. It might be called the 1 Series M Coupé, but underneath it's an M3 and on top basically bespoke, the body stretched to accommodate a track that simply wouldn't fit under a 120d. Trademark M Division parts - twin stalk mirrors, quad tailpipes, M3 CSL 19-inch wheels - finish off the aesthetic. It's a glorious, boil washed touring car, but one that happens to fit four comfortably and have a decent boot.

Behind the Wheel:

Compared back to back, you can't help but leave with the feeling that the perfect cabin would be a meeting of the two in the middle: the 1 Series doesn't do enough to feel truly special - as lovely as the orange stitched Alcantara trim is - while the Cayman is so uncompromising that it would take a driver of uncanny patience, physical flexibility and partial deafness to live with it every day.

We're sure it doesn't spoil the conclusion to say already that the BMW is the one you'd buy if you could only have one car - it just does such a good job at being boring. Its one-size-fits-all single-rate springs and dampers eschew the adaptable suspension trickery of BMW's other high-end stuff, but it's well judged, mitigating the horrible extremities of the road yet communicating the essential information.

Good job too, because it's lively. All 369lb.ft sent very quickly to the back wheels means the BMW demands respect. That said, it does seem less willing to let go of its back end than an M3, and the relative closeness of the driven rear wheels to you (whose width and negative camber are a sight to behold) imbue a feeling of being that little bit more in control. With the traction control on, hot hatch style liberties can be taken with this car - though switch MDM on (that's M Division Mode) and it's very easy to get the back wriggling.

Not something that can be said of the Cayman, whose mechanical grip levels operate on a different plain. So much so, in fact, that its mid-engine, rear-wheel drive layout almost becomes moot; the Cayman's limits are so high that, on a public road, breaking them seems near impossible. It has that Lotus Elise-like quality of putting up a paper-thin barrier between driver and road. Sharp and uncompromisingly unfiltered.

The BMW is undoubtedly the quicker car in real life, due to its ferocious mid-range, but anyone can jump into the Cayman and quickly start dismissing corners with exorbitant, almost reckless abandon. You simply can't do that in the BMW.

Problem with the Porsche is that, yes, it's more fun when you're driving, but the rest of the time it's a proper pain in the arse. Wedged into the fixed position carbon buckets, you'll feel you've strapped on a muscle toner belt - good for shrinking those burger wings, but not really that pleasant after a while. It's also a cacophony of raw engine and tyre noise. Pure indeed, but probably a little too pure to live with all the time.

Money Matters:

BMW has restricted UK buyers to just 450 1 M Coupés, and at £40,020 it's a good £14,000 cheaper than the M3 on which it's based. That's probably why a third of the allocation had sold before the car went on sale, and why we think this car represents one of the best investments you can make this side of a supercar.

Porsche will argue that the work is has put into shedding weight from the Cayman S justifies the circa-£4,000 premium, but for us it seems a little rich. Some of the work is custom to the car - the tweaks that extract an extra 10bhp from the Cayman S engine, and the rear spoiler, for example - but Porsche has essentially stripped the day-to-day usability from the car, meaning you'll have to have something else, kids or no kids.