I think a lot of the difference is in perception. The M3 sounds quick because that is partly how it is designed, the 335 is a lot quieter in everyday use and its speed is more suprising. Do you get what i mean? I have driven countless M3's and loved every minute,they sound "fast" I dont think M3 owners and 335 owners will ever agree on this but they shouldnt have to. Its partly BMWs fault for releasing a car that could cause this discussion in the 1st place.
I came from a very highly modified Focus ST to the 335 and thought the 335 was slow, it wasnt,it just sounded like it was.