It all depends what you're after; technically the smaller rims give you better acceleration, but the larger ones give you better top-end speed. Obviously the smaller ones look a little lost in the arches, but larger ones can catch (especially if you've got dropped suspension... maybe so the small wheels don't look odd). - Quite a few people run 16"s as that seems to be a good compromise on overall radius and clearance; they should even clear most big brake kits.