one thing to take into considerration.
multipiece rims look smaller (rims where the 'spokes' are in the 'middle' of the rim, instead of on the outside)
a multipiece 18" rim looks more like a regular 17".
so..if you are going multipiece, i'd definately recommened getting 18s because of this. (because getting multipiece 17s will look more like regular 16s).
its obviously just an optical illusion...but looks are what wheels are all about for the most part (sometimes they're also about weight)