Read Phil Floraday's comments on driving the 2008 BMW M3.
I'm sure the M3 is faster around a track than a 135i or a 335i, but I couldn't tell much of a difference on the street. I really noticed the lack of torque compared with BMW's twin-turbo six. Torque peaks with 295 lb-ft at a rather high 3900 rpm in the M3. With either of the twin-turbo cars, the torque peak is at a lowly 1400 rpm and there's at least 5 more lb-ft of twist as well. That difference in torque wouldn't be a problem on the track since the M3 revs sky-high.
Perhaps an M3 equipped with a DSG-style transmission would suit me better than this one and its six-speed manual. I had a hard time executing smooth 6-4 downshifts for passing on back roads. I kept looking down to make sure I was hitting the right gear because the shifts are a little vague. A couple quick taps on a paddle shifter would make the transition foolproof and the revs would be perfectly matched. Maybe I just needed more time to get used to the transmission.
The $60,000 question boils down to C63 AMG or BMW M3. For my imaginary money, I'd have to pick the Benz. I had a few complaints about the seven-speed automatic in that car, but it looks more menacing, barks louder when you boot it, and it absolutely rips when you get on the gas. In a perfect world with no traffic and smooth, twisty roads, the BMW would probably be a better choice, but there's a lot of traffic and long straight sections of road in my life. The Mercedes C63 AMG just fits my life a little better.