17's are better than 18's performance wise.. ask anyone who tracks their car..
Also, i dont know where you live, but around here 18's would get destroyed in one day driving around town (bumps/potholes)
When i went to get one of my bottlecap's straigtened a while back, the guy who owned the rim shop said that there was a kid in the area with a BMW that comes to him at least once a month to get his 18's straightened.. that was the main killer for me..
Anyways, your choicE!1!!
ps. contours have to be the easiest rims on earth to clean.. come on, you kno you want em