Car and Driver also states the M5 is a hell of a lot cheaper than the M6. Somewhat misleading, these are both $100k+ cars when all is said and done although the M6 base price does start at $106,100 versus $89,900 for the M5. Hell of a lot cheaper? Neither is cheap and the difference becomes negligible with options at this price level. The M5 certainly is no bargain.
These two cars are pretty much the same thing. The M6 has a lower center of gravity and is slightly lighter at 4088 pounds versus 4313 pounds for the F10 M5. That is really what the difference between the two comes down to. Less weight means the M6 edges the M5 in acceleration at 8.5 seconds to 100 mph versus 8.7 for the M5. It also runs through the 1/4 mile at 2 MPH higher at 124 versus 122 for the M5. Keep in mind the cars were not tested on a drag strip so those trap speeds are likely inflated.
The M6 is not surprisingly also better on the track. It hit a higher peak speed and was quicker through every sector. The difference is minor just as in acceleration but the M6 is the better performer of the two. If one wants the absolute best performance, the M6 is the choice. If one wants four doors, the M5 is the choice. They both perform admirably.