Having in owned both and currently in a Cayman R I would say without a doubt the Cayman is a sports car. Seats two. Very light. Very responsive steering. Rides hard. Is loud.
The M3 is a fast GT that handles well. Very different cars and not really comparable. If you like comfortable and quiet get an M3. If you like hard edged get a Cayman.
As far as 997s go-they are nice but to me they feel more GT than sports car. The last 997TT I drove on track felt just like my M3, kinda fat. A fast kinda fat