For me, I consider the M5 and M6 in different classes. The M5 is a practical sleeper of a car that can comfortably carry four adults and doesn't try to be too sporty/flashy. The M6, especially the Grand Coupe, competes more with the 911, R8, RS5, etc.
I think the M5 is still the best car in its class, but I would not personally choose the M6 as the best car in it's class.
That said, if you don't have room in your garage for a complementary sportier car, don't need four usable seats,etc.-- you might pick the M6 as the better all-arounder.