I can backup mfstout's statement that the ride quality is not affected one bit on 18" alloys. I did have my doubts. I was originally going for 17" alloys for ride quality, but deep down I wanted the 18" alloys.
However, after I test drove one with 18" alloys. I, too can confirm that it does not affect ride quality. In fact, I was amazed that it didn't.
18" alloys all the way. looks better and doesn't compromise the ride.