I think it all depends on a number factors: money no issue and you wont mind depreciation of a new M3 but get all the options you like or you want to save some money and get a slightly used one(end up trading out some options you may want to have)
If you go the used car I suggest a CPO with longer warranty.
Drives: 997 Porsche turbo
Stable: Porsche Panamera
'09 e93M(sold), '09 e90(sold)