You know I think the M3 does a wonderful job of making you "feel" like you are going fast, whereas the Cayman tends to hide it. The test numbers don't lie and my own personal track times don't lie either, stock for stock the Cayman S is as fast or faster than the M3. Check the magazines for some numbers if you want, but I also have real track comparisons where I've gone up against multiple M3's both when I was stock and after my mods. In stock form in straight line acceleration I was always just a hair faster, not much but enough to see me pull on the M3 a little as we went down the main drag straight at HPT. In the corners it was no contest, the Cayman S eats a stock M3 alive in the corners.
Ok maybe that is a bit of an exaggeration, but clearly the Cayman S is better in the corners.
The thing is I would teach some of these guys in M3's (was just with the BMW club this past weekend driving the new config at HPT) and you'd swear you were going really fast, climb into the Cayman and while it didn't "feel" as fast or maybe "sound" as fast, the Cayman was quicker.
Of course either car can be modded but I had a Dinan modified M3 tell me at the track this past weekend that my
Capristo exhaust sounded great as I flew by him and he couldn't catch me.
So it may come down to what you are looking for. Nothing wrong with the M3, great car and certainly feels and sounds "fast" regardless of what it is actually doing. I understand the new one is quicker straight line than the outgoing model but BMW fans have been lamenting its continued weight increase and lack of handling and track precision.
It really is sports sedan vs. sports car, two different animals two different approaches. By all means buy the one that best suits YOUR needs. We aren't going to put you down for choosing the M3 if that is what excites you. Get out and drive and have fun and if you get a chance to drive in or ride in a Cayman at the track, take it, and you'll see what was missing from a dealership test drive.