i've always been partial to a spoiler myself. it seems to break up the rear a little bit. but not having a spoiler does make it look a little lower and streamline. either way, it's an M3 and who couldn't like that?
only the oem looking molded ones. everytime i see an aluminum wing on a street car, i just want to slap the person driving it. if i had to make a choice though, i'd say spoiler.