It's a valid question. The REAL short answer is, you're going to get F'ING ripped off buying tires for a 19" wheel as opposed to an 18" for F sake!! Stupid 19" wheels. Not only that but performance wise, if you're looking for improved performance, a lighter weight (18") wheel would be better along with a bit more rubber on the wheel rather than just a condom thin rubber wrap for crying out loud. Something tells me 19 & 20 inch wheels aren't being used by professional motor sport race teams. I could be wrong. Nascar's doing it with like a 15" or something. Lotta rubber there... haven't heard complaints.
I don't know when donk became all the rage for "performance" cars but it is interesting to say the least.
To answer your question, it's not about the wheels. It's about the tires on them. Personally, I like the look of a little more tire anyway under there.
To everyone else that lent there opinion of not buying an M3? Really?:
Thank for your technical input.