Hey Andrew, I have owned both an e90 m3 and a w204 c63. I must say these cars are completely different. To start things off, the m3 is super direct, responsive and has incredible steering feel. The handling is almost video game like, it is very well balanced and easy to control through its limits. The handling is on point, my only complaint came after driving the c63, the m3 is almost too easy to drive. As for daily driving, I was running 17mpg which isn't bad and no power at low end so if your doing a lot of city driving its annoying.
Moving onto the c63. Tail happy. Brute Power. The c63 is simply a beast. The torque separates it heavily on the straights. You will have a lot more straight line power and potential for more with a few bolt-ons. I dyno'd at 450whp with a tune, secondary cat and resonator delete and green filters. As for steering/handling feel, the c63 does have more powerful brakes but will not be as direct as the m3. This car is harder to drive through its limits and obviously does not handle as well as the m3. As for daily driving I find the c63 to be the better option, it is more composed on the highway and easier to drive at slower speeds. (doesn't feel like a race-car aka m3) The 6.3 engine drinks gasoline though you will be filling up more often I was averaging less the 13mpg in the c63.
The m3 is a drivers car, at times it felt to me that it was too easy to drive though. The v8 sings at 8400rpm and the DCT transmission is a huge +1. Torque is very low and no low end power gets irritating in the city. Superb handling and the steering is fantastic.
The c63 is a muscle car. It will eat up supercars on the highway with the simple mods I listed (I ran an 11.8 @ 123 w/o tune). Better for daily driving use and comfortable on the highway. Transmission does not compare to dct, it is slower. Drinks more gas than the m3. MORE FUN because of the rawness factor.