[wzj519]

As you are looking at both M5 and M6GC, I assume you need a four door car.
I didn't have M6GC before, but had 650GC. When I had that car, if I adjust driver's seat to my comfortable seat position, then there will be impossible to have an adult male sits behind me. For your references, I am 6'11 and 200lbs.
The M5's rear seat space was also not that kind of good, if I am sitting comfortably at my driver seat, then there will be not much leg room left for the driver side passenger.
Personally, most people go for m5 for space and convenience, and choose m6gc for styling reasons.
If you need carries four people always, then M5 might be a better choice. Otherwise, if you just need a four door occationlly, M6GC would be the ultimate choice.