Do some searches and you will find a lot on this subject.
Bottom line:
- 15" - Pros,original and in my opinion, better appearence. Con, $hit for tires
- 17" - Pros, great tire selection. The difference behind the wheel is nothing short of amazing. Cons, Non-original appearence. They don't 'fill' the wheel well quite as nicely. I have seen some with careful tire selection and ride height adjustment that in my opinion don't look bad but I have seen many more that to my eye look like the tires are undersized.
To me it comes to which is more important, the experience behind the wheel or the experience standing and looking
Rick