I have to say I disagree with all the above posters except GTIain, driving my ed30 with 18" pescara's back to back with my wifes gti on 17" monza's both with quality tyres on, the ride is noticebaly harsher on the 18's (i assume the suspension is the same??) also the handling on the 17's (on the road) is better, sharper turn in and agree with rr you can feel the weight difference.
18's harsher, more expensive, handling not as good (on the road) but look better. I suspect that the 18's will be better on the track as less tyre roll due to stiffer sidewalls, the handling on my track car with Michelin Cups is rough on the road but sublime on the circuit because the side walls are like Iron and you get no side wall roll.