Parked right in front of a GTI this morning with the 18s on and took the opportunity to eye it up right next to mine (which has 17s). I honestly couldnt make up my mind which I preferred. The 17s are more simple by design which gives it a bit more "grown up" feel, but I do like the design of the 18s.
I think the fact I am even contemplating which is best suggests I am not 100% happy with the 17s, so it might be a purchase in the future

Then again dont intend on keeping the car that long and dont think it will put much on its resale value