I faced the same decision and decided to go new. I was a bit worried about how a used M3 may have been used by the previous owner. Was it tracked? Did he really like launch control?
With a new car you don't have to worry about abuse and you can get exactly what you want. If you can afford new it is a safer option. I normally buy cars a year or two old to avoid some of the initial depreciation, but decided to buy new for the first time in 9 years this time around.