I think most people who say M5 "because it can carry kids" are just carrying mainstream words. I mean come on, how often do you drive your whole family in your M5? If too often to choose an M5 - then you bought the wrong car. Better choice would've been a 5 series and a dedicated 2 door sports car.
I think the preference is based on age and how men view themselves and their image. A 30+ years old man coming out of M6 can look "show off" or awkward at times. I've had M6 and now M5 F10 and I find M5 to be the best overall car in the world, and fastest sedan in the world. M6 is far from being fastest coupe in the world (not even in top 20 probably).
M5 is more a car for "how you see yourself", M6 is more of a car for "how others see you" or "how you want others to see you".