M3 if you're after a pure driving tool. The new 6 is blighted by indirect and soulless steering (because of active-steering), harsh run-flat tyres, and a silly manual gearbox setup (mainly due to CDV).
The M3's driving characteristics COMPLETELY dominate the 6er. IMO, the 6 is for oldies that want to drive long distances in comfort and at a quick pace. The M3 is for the real driving enthusiasts. In this instance, I wouldn't even consider, for one second, plumping for a 6 over an M3. It's a no-brainer really.