[diononiz]

I was deciding between the two and while It was a very difficult decision, I am very happy. Here are the factors (in my opinon so your mileage may vary)

Exterior:

I thought the C63 looked really nice and aggressive but it still looked like a regular C with added AMG bodykit. I really liked the C63 exhaust and rear diffuser. Overall I preferred the design of the M3. It just seemed better integrated and flowed better, to me a more unified design The M3 looks a bit less "mean" but still very aggressive. I really like the front bumper design and side profile.

Interior:

I really dont like the shiny plasticky interior of the C63. Honestly, I felt it was a step back from the previous generation. I did like the pop up Nav screen and the seats but overall the interior was a slight downer. The M3 looked better to my eye. The interior just seemed more upscale and the seats are very impressive with more adjustments.

Engine:

While I enjoyed reving the M3 engine and appreciate the power it makes, I really like the torque and exhaust sound from the C63. That engine is a beast and the torque is just awesome.

Suspension

Call me soft but coming from a 535i I found the suspension a bit too firm in the C63. The adjustable suspension in the m3 I found to be the perfect blend between sportiness and confort. I also found the C63 a bit heavy feeling and didnt feel as nimble.

X factors

Both cars have a great incentives right now (especially the C63 with 7500 off) but the included maintence for the BMW is nice. My father in law has an AMG car out of warranty and they are very expensive to maintain. These cars are so close that I had to consider the little things. The BMW was also available in a manual transmission. I had the Sport Automatic transmission in my 535i and although it shifted very fast, I was honestly bored. The M3 is also available in a convertible. Yes I know performance is going to be degraded by chopping off the roof, but since I live in CA I was willing to sacrifice some performance for the open air motoring

I ultimately decided on the M3 convertible. Although I miss the engine and the straight line speed of the C63 (and the coupe and sedan version of the M3 for that matter) I am very happy with my decision. Since I personally dont drive that aggressively on the street, the m3 cab has more than enough performace for me.