Again imo, The BMW M3, has an engine, which is just not a match for the C63 and sounds, err, rather disappointing. Its brakes are at best just about satisfactory for fast road driving but do not inspire confidence of the RS4 or the C63 AMG PP. The ride and handling are very good though.
With the C63 AMG, what sold it for me was the naturally aspirated 6.3 litre V8 engine, which is an absolute peach and the sound it makes, which is awesome.
The Performance Pack upgrade gives it mind blowing performance and the knowledge that the uprated engine internals are up for it. The uprated brakes inspire confidence and the ride / handling is good.
Yes, there is a potential fly in the ointment with the head bolt issue on the pre 2011 cars but this can be rectified for about £2k IF there is ever a problem.