While the Turbo & GTR are stellar cars I would get tired of them after a while, with the M3 supercharged you get the best of both worlds. So I would choose the M3 hands down! Matter fact I passed by the BMW dealership today on my way home & there was a super clean 911 parked for sale that someone had traded in.
A M3 is a hard car to beat in real world driving not to mention a supercharged M3!
However, if you're looking for an all out track car I would lean towards the turbo.