It depends upon what you want...
C63 crushes the m3 in a straight line and will be more rare
M3 is the better handler and is faster around a track
Both will offer a dual clutch transmission and can be had in sedan form and from the initial info looks like the pricing is very close. The exteriors are pretty equal (based largely upon personal preference) but I think the m3 interior is much nicer. If it matters to you, the M3 will get better gas milage as premium gas is now $4 per gallon. The C class I drove at the C event hosted by Mercedes had a very cheap plastic interior with a few rough edges. While the m3 has a great exhaust note. I feel the C63 has a better one with a deeper more aggressive growl.
Either way, lets face it...how can you go wrong with either. I am going to drive both then make my descion.