i have a GT3 and an M3. i'm no race driver or anything but i would give a slight edge in throttle response to the GT3 (although the M3 power button closes the gap some), but a huge advantage in 'fun' factor to the GT3. to me, a fun car that has great throttle/steering response, is wonderfully communicative, and just works seamlessly with its driver. the kind of car that makes you feel like a much better driver than you are. oh, and it has to be as fast as fuck around a track too
the GT3 is better than the M3 for me at all of the above (although i've not personally taken either to the track yet). but the comparison really isn't fair because Porsche set off to build a track car with minimal compromises for the street, while the M3 was designed to do all things well. oh, and the base price of the GT3 is literally double that of the M.
i will always grab the keys to the GT3 if i'm in the mood for a fun drive, but there are definitely times when i feel more like driving the M3. i do feel the M3 offers a ton more comfort and convenience (with DCT) while only sacrificing a little of the fun factor. bang for the buck, M3 wins by a mile.
as far as Corvettes, i've never owned one so will withhold comments. i drove a Z06 and it was definitely fast and 'raw', but not my cup of tea.