I'm thinking about going to a 15 in. wheel instead of my 18 in. I want a wider tire/look. Anyhow, I've offroading the past 4 years in Moab and 18 in. wheels never touched the rocks anywhere. With that said, Billy is right. You would have that much more sidewall with a 15 or 16. That would equate to a bit more protection.
Peace.
Tom
012009