I still don't get the question. Why would one be "better" than the other, it's all about "needs". If you need to haul around a bunch of people, the M5 is better, if you don't and like a coupe with the longer window and doors the M6is better.
It's like saying "which is better, e90 or e92" they are both great and I don't think either is "better" than the other, it just depends on what you need in a car.
'14 528-xdrive - daily driver/winter beater, '06 Boxster - for fun, '07 M6- for even more fun.