Depends what you're looking for...
I drove both for several weeks. M3 is especially a good car if you're looking for a complete performance/luxary car. People tend to look at 0-100 km and 1/4 mile ; message to those people : speed is a factor but it omits several factors ; gearbox, suspension, road feeling, handling, braking, re-acceleration, sound, etc. M3 got it all. However, comfort level in a M3 is similar to an Acura Integra (not that bad but almost!) ; it's noisy, bumpy, not enough room for passengers, etc.
C32 AMG is a fast car too. Between you and me, WHO CARES if it's faster or not than a M3 ; it's a FAST car, point. POINT. However, the C32 is more of an aggresive car than a sport car. You get luxary, space, comfort, etc. The C32 is a better "good father / good family car" than the M3 ; it could serves as the main car ; the M3 is not ideal for this purpose. The C32 is amazingly fun to drive but you don't feel the road like a real sport car driver would wish...