Here is a link to a fairly detailed step by step method of fixing scratches, nicks & other paint problems. It uses factory touch up paint, several steps of light sanding, buffing & polishing. But the results are quite impressive. Complete with pics! (It is from that 'other' site).
http://www.bimmerfest.com/forums/showthread.php?t=32625
I have a few minor nicks I need to take care of too & will be trying this soon.
Problem is, if you don't have all of the stuff (polisher, polishes etc), it can run quite expensive if you're only going to do it once. However, if you are a detail freak like me, you already have all of the stuff & it's a no brainer.
Getting your bumper repainted is a dicey proposition. Most aftermarket bumper repaints I have seen have flaked off after a minor shopping mall bump. I'd rather take my chances with the touch up paint method & know that I can redo it at any time.
My opinion...
Johnny