I've owned both. The 335 is a great car, lots of fun and easy to mod, better mpg. However, the M3 is a totally different animal. V8, M suspension, LSD, different interior and exterior parts. M cars are rare in CT whereas 335s are everywhere. Depends on what u want obviously but you really can't go wrong with either car.
Drive one and you'll know. I never know until I drive em. GL