the cayman does look good and sounce really nice with aftermarket exaust. but it is kinda small for my taste and not really practical.
depend on what you need really.
If I'm going to get a new car in the same class as the m3 right now, i would go for the c63 amg because in canada it is 10k cheaper than the m3.
I like the c63 big engine (a tune away and you have 481hp). paint the brake red and add some carbon parts, would look sinister. I like the new interior in the c63 amg more as well.