personal choice...Standard STi had 17's - cheaper tyres than 18 but aesthetically don't look as good. I still have my 17's sitting in the garage as after I fitted my 18's I figured it might be worth holding onto them.
18's look better, and give more tyre on the road (providing suspension set up correctly) but tyres are dearer.
I think either can look good if suspension height set up right, and car handles well.
Probably not the technical answer you wanted, but that's my two pennies worth! ha ha
