I can’t really convince you to get either one but I can tell you my experience when I was buying my car few weeks ago and why I picked the M3
We compare the two cars C63 vs. M3 but they really are 2 different cars.
C63 is a great car, the engine and the note is just great and crazy fast, but few things I didn’t like about the Benz was its Automatic only, the interior those feel cheap, (but the seats are very nice) and I didn’t want a 4 door.
The M3 is faster on the track and slower on 1/4 but that doesn’t matter that much because I’m not a professional racer and the difference is very small it’s more the feel of driving the car everyday that matters more, I got the E92 because of the 6MT, the Fox Red
, and two door, and the M has a more of a sports car feel (You already know that)
I think the M3 has a more of feel of entry level to exotic car, especially with the Red seats, while I think the C63 is just an upgraded C class.
I personally spent some time with both cars, because in the end it matters how you feel in it and what you like and is more important to you, you know that M3 since you had one, go take the C63 for a drive sit in it and see what you like more, I’m sure you'll know the answer to that whit out anyone convincing you either way.
In the end both car are absolutely great but very different, so it comes down to what you want and personal preference.