Dude this question has been addressed before, kind of. Or at least addressing the price difference.
As for the difference in the number of buyers between the two. I think it has to do with price and practicality as the 2 big reasons. I could afford either, but I decided to go with the M5, even though I am single, have no wife, no kids, no dogs, no cats, and no goldfish.
I think the M6 is too big for a 2 door car. The M6 reminds me of the Mercedes CL65, a car that to me, was way too big for a car that only has 2 doors. I like the tail lights better on the M6 than on the M5, but I hate the interior. I don't like the stand alone design of the iDrive screen in the 6 series, but some weird reason I think it looks good in the F30 3 Series. For the M6 I also don't like what they've done with the side grills. And that angle that comes down after the side grill on the fender really bothers me also. Makes it looks like a Mustang to me for some reason.
In the end I chose the M5, because it suits me better. I like the interior better, I can fit my friends in it easier, and it was more of a bargain super car. The only thing I really want from the M6 is the adjustable side bolsters. For me, the 2015 F82 M4 Coupe will be the biggest 2 door coupe high performance car I would ever get from BMW.
However, I will say that the F12 M6 is in fact decently faster than the F10 M5. That extra lightness of the F12 M6 vs F10 M5 makes more of an impact than between the E63 M6 and the E60 M5.
Here's what I wrote previously in the other thread.
Here's what you get when you buy the M6 that the M5 can't get:
- Carbon Fiber Roof
- Shorter Wheel Base
- About 150lbs lighter
- Carbon Fiber Trim Option
- White Ash Grain Wood Trim Option
- Adjustable Side Bolsters
- LED Headlights
- More sporty Steering Wheel
- San Marino Blue Exterior Color
If you buy an M6 here's what you can't get from the M5:
- Rear Seat Heaters
- Power Close Trunk
- Aluminum Trace Trim Option
- Monte Carlo Blue Exterior Color
- 6spd Manual Transmission
That's about it I think. So you can see the difference where that extra $16K comes in.