I would buy the new M6 if that was an option for me. Twin turbo torque is awesome. Factory warranty. Comfortable, good looking car. Might not be as nimble on a racetrack, but I'll guess you don't spend much time on one so that does not matter. Even if the M6 is heavier, I think it would beat the M3 on the track.
Since the M6 is not an option for me, I may supercharge my E90M3 at some point with a used kit.