335i has beautiful modding capability. Truly amazing and if I were in your shoes I would be tempted so much with going with the 335i. However after the many reviews I've seen people say that the m3 is just "different". Sorry I can't give you real experience as I didn't go from 335>m3 but the m3 is worth it. If you are a performance oriented person trust me when I say you will get bored with the 550i and especially the 640i. Great cars and I would like to own one in the future as a daily but you will just get bored after awhile. The m6 IMO is a bad choice just due to costs and the fact that its now outdated.
10 E90 M3/dct/ztp/zpp/heated seats/eps/19'/iPod
retired- 08 535i/nav/prem./L7/ipod
retired- 06 325i/nav/prem./sat/xenons/style 128