well I have been driving a C63 for a little over a year now and have had the chance to drive an M3 coupe (09) many times during that year (father has one). We both agree that they are both outstanding machines and both of us would be happy with either one. I am normally a BMW fan, but couldn't pass on the discount Mercedes was giving on the C63 ($14K off, brand new).
anyway, the biggest difference we could tell between the cars was feel of power/speed and agility. Driving the C63 everyday and then getting into the M3 I can say the M3 feels lighter and more agile even if its only 275 pounds lighter. The C63 on the other hand feels a ton more powerful even though they are similar/same at 0-60 times. The extra 37 horsepower might just offset the weight but where you feel the power is teh torque, it is about 440lb compared to 300lb on the M3. My dad was even shocked at how different the excelleration felt, he thought for sure my car was faster because of that torque power feeling. So when you see that the C63 can pull off 3.9 or 4.0 and the M3 is normally around 4.2-4.3 depending on the magazine, that is why, yeah its heavier with only 37 more horsepower, but that 140lb of extra torque goes a long way. If I could buy either car at the same price it would be a tough choice, I am so in love with the seats in the AMG, they are better than the M3 and overall driving experience is great, but I still love M's (have owned two in the past). One thing is for sure, I would take either of them anyday over the Lexus or Cadillac, they are much nicer cars all around, Lexus is too new to the high performance game and Cadillac still feels cheap inside to me. And I don't like how they use the 3.9 time in their commercials, all car manufacturers should use "real times" like the Germans do
you don't see BMW with M3 commercials saying 4.1 seconds 0-60...
All that said, I can't wait for the new crop of compact performance cars from Mercedes and BMW, now it will be a MPG war instead of power war, I think the 4.0 second time is the standard and no reason to better that in a car that cost under $100K, now get us 20 mpg on average instead of 15-16!