Nice car!
I agree the GTI looks at it's best on the 18 inch Monza II (or Detroit as called here in NL). However the inch Denvers actually appear to be the more practical choice. They lend them a bit better to inner city abuse (kerbs etc), have a lower unsprung weight., and a bit more comfort whilst remaining the suspension and handling advantages.
I won't get the infamous EVO tyre test into the discussion, as there are also a few tests claiming the 18's produce faster times.
For me this, and the fact that I was approaching my max budget, was the reason I selected the Denvers.
However, I really do love the look of the Detroits more every day and have been making some inquiries at my lease company if it is possible to buy a set of them and add these in the contract (ie higher payment due to more expensive tyres, possible replacement and damage repair etc). No luck sofar,