No comparison between an e9x M3 and 550. Apples and oranges. I test drove the 550 with sport package and it's handling + steering was like crap. Also, it's grown too much...too heavy and unbalanced. It has plenty of power and is a beautiful car, but in my opinion it's a major disappointment. Performance wise I would prefer the e60 with sport package. It's a more confident car. If it's a sports car you want and something that actually drives/feels like a bmw, get the M3. If you want a lexus like luxo cruiser, then get the 550. By the way, the M3 just beat out the new RS5 and CTS-V coupe in the Car and Driver and Motortrend comparisons.
http://www.m3post.com/forums/showthread.php?t=413025
http://www.m3post.com/forums/showthread.php?t=411419