Love both cars, if its just me and Lucy, it would be the Cayman, if we are lucky and have kids it would be the M3.
Just as a left field choice have you looked at the x150 jag, that's the new shape Xkr. Easily in budget, its not as much of a barge as the old x100 shape like mine. Plus the supercharged 5 litre v8 is very good. Loads of creature comforts. Even nicer place comfort wise than the Cayman. And you can get 1 mate in sideways along the back. The convertible version is even more fun.