Show or go?
If you want show, fit 17s. Bigger wheels look good. People who don't actually know anything about the subject, ie, most people, think they're better. So if you want to impress most people, that's what to fit.
If you want go, fit 15s, they're lighter. Lighter wheels do everything better.
If you don't know, can't make up your mind, are indecisive, fit 16s.