I am looking at a mk5 gti and have managed to drive cars with both 17 and 18 inches. It was hard to tell if there was any major difference, both rode well, but crashed a bit on slower poor quality roads. I really don't think you could tell the difference, or if so, only very mild.
The only negative I see for the 18" (aside from initial cost and tyre cost) is the laquered finish. Which looks great new, but ALWAYS has problems later with water damage / peeling / needing refurb etc. But I assume they would be replaced under warranty? so what the hell!