Some folks don't like to make serious modifications to an already great car. I want my car to be fast right off the showroom floor. I can guarantee the M3 will be faster right out of the showroom. I personnally don't care if some dude spends $15-20k on his 335 to make it faster than my M3. I know my M3 is still covered under factory warranty, will probably be less of a maintenance headache (than some finely tuned turbo car), and will probably last longer 150K + miles. Plus, I will still be faster on a road course in my M3.
I am thinking 3k-5k is going to give you one hell of fast 335 given that Xede tuned 335's are doing 0-60 in the low 4's and 0-100 around 10 sec. For the street only its going to be hard to deny the 335 may be a better route for the guy that doesnt mind a tuned car and no warrenty.
I would rather have an M but if the price is wrong then it looks like I will be waiting for the 335 M sport with a Xede or at that point a Dinan tune to chase down Porsche's with.