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.
'16 - 435 x-drive coupe, '14 528x-drive, '07 M6 (I am a sequential BMW hoarder)