I used to have an e92 m3 with DCT that i sold back in 2010 and had a bunch of cars since then including an ISF. I have not missed any car like i missed my M3, and last august i traded my ISF for a low miliage e90 m3 with DCT. Went with e90 since we have a little one now and another one on the way in a week or so
The ISF was nice, but the M3 does everything better imo. The ISF is a hooligan, a butcher's knife, while the M3 is a brain surgeon's scalpel. The only thing i miss in the ISF is the yahama tuned torquie V8, but in my opinion the s65 is way more fun!
I have not owned a C63, but i have driven one. It's crazy fast, sounds great, and that's about it. Not in love with the interior or exterior styling, and though the seats have better support, they are hard to get in and out of. So if you're 6ft+, it might feel a little cramped in there. Hope this helps and good luck!