Should I do aftermarket rims?

So the tires are finally going to be up on my 2013 Prius C Two, habanero. I have pretty much stock everything except I did add the extendmyseat brackets to the driver side (TOTALLY WORTH IT!). I am thinking about doing aftermarket rims simply because the hubcaps are so boring. I would love OEM, but I think the aluminum rims are only 16 inches right? I want to stay as close to stock as possible to maintain my great mpg. It looks like you don't have much choice but to widen the rims to 6 or 6.5 inches. I am thinking 15x6.5. Anybody have experience with the effect on MPG? I live in a pretty good area and plan on using wheel locks, but are 15 inch rims less likely to be ripped off? Tire wise should I move to 185 vs 175? I have seen some people say you can mount the 175's on a 6.5 inch wide rim, but others say it isn't safe. Would be interested in what I should consider before making the plunge.