From all the reviews I have read before I got my GTI, and test driving a car with the 18's on,
Harsher ride if only slightly with the 18's, along with less handling and acceleration. Also 18's would be prone to more kerbing damage as they have less tyre sidewall, 40 series profile as opposed to 45 series.
I have to admit they look better, but after driving both quite spirited, I preferred the 17's and saved over £400.
Oh and I don't believe the 17's get white worm, or am I incorrect?