This test would reflect my experience relative to what the difference would be... C63 is notably faster, M3 weighs less and this would be noticed on the track. On the back roads or streets, the power of the C63 would be great and the handling is very, very good (as C&D noted). The advantage of the M3 is at the track (not all tracks would favour the M3 but likely most) and that is where the chassis and weight difference shines. Street and back road driving where power/torque are important would favour the C63. Wanting a car to go to the track every weekend would favour the M3. Both are great choices and the best in their category.
Very similar cars but each with a slightly different flare. One point spread is as close to a tie as it gets. Also, if you compared two similarly equiped cars, the M3 would not have had a 4 point price advantage. Got to give the C63 props for power/speed... 4 seconds faster to 150mph is significant. Also, 12.1 trapping 120mph in the 1/4 mile is very fast for a stock car.