The M3 and Cayman, although in similar price ranges, couldn't be more different to drive. We have a 2011 M3 sedan with DCT and a 2012 Cayman S with PDK.
The engine in the M3 is a beast that just keeps pulling, but it's still a large, heavy sedan compared to the Cayman.
The Cayman, although lacking somewhat in ultimate power, handles sublimely and is fun to drive.
I had my Cayman on the track last weekend, it's amazing how it will keep up with the Carrera's on a tight track like we were on. This was only my second time on the track, but the experienced Cayman drivers were just blowing by me, on my butt on the curves so I just let them by (this was by no means a race). I haven't had our M3 on the track, but I can't see taking corners in it like I was in my Cayman. The PDK transmission in sport plus mode turns it into a different car, holding every gear until red line and rev-matching downshifts when braking into turns. The engine is so different in the upper ranges of RPMs.
I have no doubt the 981 will be a brilliant car.
Don't let the starting price fool you though, it's easy to put 20k or more in options on them without even trying very hard.