the problem with the m6 is the m5.
m6 costs more, is heavier, not as quick.
if things go wrong, well, it's an M car, so yes it's not usually cheap. front brakes, save over a grand for when they need replacing. that's just a taste of M car ownership.

M5 will be cheaper and as nice if not better in my opinion, i'm ll about getting an M3 at the moment, more of the handling for me.