21's look really nice.

Get the 20"s. I actually prefer the ride with the wider 20" than the 18s or the 19s. If others judge you for that, tell them that handling is improved with the 20"s. Most people don't even realize that the 20" are optional features. they just assume that bimmers naturally have dubs on (especially if they are naive enough to judge you based on the size of your rims )

P.s. My motto - Do the best, forget the rest.