Interesting topic, and I can say for me my first choice was the M6. When I considered what I needed the car for the M5 was the clear choice for the following:
1) Performance is relatively the same no need to split hairs
2) I own a 650ci, and yes the back seat sucks but a sexy car
3) M5 more room again, and same performance and the same options
4) Sun roof, and more room for trips golf clubs and bags
5)6"3 my heat hit roof at time in the 6
6)I would have purchased a 911 vs the 6 due to size so the M5 works for me.
Those were my primary reasons, and the 6 series is a lower production car across the board hint inflated cost. Non M cars the 6 comes with better options that are options on the 5. But I will say I hate the NAV on the M6 is looks after market. However the seats are kick ass along with the steering wheel!