Couple of thoughts...
1. If you drive both you will know which one you prefer and should own. Who cares what we think.
2. As noted, too bad it doesn't come with the P31 package
3. Something is odd with the pricing. A new C63 is as much or more than an M3 in the US. How can you be getting an even deal. Is the C63 a demo or used? I don't see how the deal is possible otherwise... a USED M3 with minimal options has to have less value than a NEW C63 with decent options.
4. I would not trade the M3 for a C63 just because of attention the car is getting here. If you don't find it compelling after driving it, keep the M3!!