I have driven C63 AMG, modded 335i, modded M3 and Here's what I think
3rd Place. C63 AMG - This what my brother drives and I drove it few times.
In conclusion, Lack of LSD. Okay I jumped into conclusion even before typing anything up because Lack of LSD is what kills C63. Really. Unless you do some wide rear conversion and slap on some 315's in the back. Car looks amazing from the front but then rear needs to be widened! if it cost more than $25000 over regular C-Class at least give'em some fat booty!
Seats are amazing. Interior isn't bad, however I do think BMW has slight edge on this. Sound is absolutely insane for stock vehicle but it is not as pretty compare to modded M3 with Intake & Exhaust. It's like Metallica vs Luciano Pavarotti. Of course M3 is latter. More defined yet powerful enough. Handling...well it actually surprised me being the fact it's Mercedes. C63 handles pretty well, but with not so good grip & outrageous power, it becomes questionable. Oh one last thing, brake system on this car is thumbs up.
2nd Place. Modded 335i - This is what I drive and I love N54.
I will take modded 335i over C63 AMG ANYDAY! that's because my car is heavily modded 335i
It's got the hp/tq that surpass C63, has LSD, pretty much outperforms C63 in every category. fully modded 335i will be about the same price as C63 AMG. Only with better mpg with less maintenance cost. For example, I took C63 in for service, $300 oil change? Crazy. If you don't care about AMG badge or Mercedes badge on your car then it's no brainer. I'm sure you know better since you own one
1st Place. E90 M3 - I took LTBMW's E90 M3 for a week.
This right here is the masterpiece. This machine handles like dream. Complete drivers car. C63 will be moved by your gas foot just like any other cars, but here's the tricky part, once it starts moving at fairly high speed, you will feel you don't have 100% control over the car. Sure it will get 0-100 in no time but you don't feel that 'Planted' feel while doing so. You should know exactly what I am talking about since you test drove one. But M3...oh man...okay LTBMW was kind enough let me borrow the M3 while my car was getting some work done. I took the M3 home and first thing I did was open the internet explorer and start ordering rest of M3 suspension bits. I always thought my 335i with LSD, PSS10, M3 strut brace, M3 Front/Rear sway bar handles freaking awesome. I believe LTBMW's M3 only has aftermarket springs on, suspension mod-wise. I don't know how much straight line during your daily driving times but think twice before you jump into C63. Even better, test drive C63 and M3 back to back. That's what I did once I got home and M3 hands down. The whole balance of the car is just perfect for everyday driving or track events. See just like the OP, english is my second language and I hate typing. But I had to write this, because my one week experinece with M3 was just mind blowing. Am I exaggerating? Well maybe. That's just how I fell when I drove the car. Planted, safe, plenty fast, sounds intoxicating with aftermarket Intake & Exhaust (Surprisngly, Intake sounds awfully good on this car, more so than the exhaust) LSD, handling, handling and handling.
Bottom line - If you drive AMG, it will make you scream OMG!! (that could be good or bad) If you drive N54, it will makes you wonder what N54 means and what it can do. If you drive M3..well...as long as you are in the drivers seat than MMM~ is all you will say