an inch...
Sorry, couldn't resist 
On a serious note I doubt you would notice the difference 'that' much from 17 to 18, less tyre wall roll as they will have lower profile rubber giving slightly better handling but mildly harder ride. Nothing to worry about, just get 19's ... 
*Cough* sorry, but didn't the GTI with 17"s on lap 2.5 seconds a lap faster than the same car with 18"s on, in the Evo tyre test?
18's look better, period. But 17's get kerbed less (more tyre sidewall), make the car handle better as it was designed to run on 17's, weigh less and accelerate a teeny bit better.
Also no white worm on the 17's, they cost less and are easier to get hold of. I chose them after finding the ride on a GTI with 18's on a little harsher than the 17's. Well happy with mine. Looks aren't everything
