On another note... The one reason I do regret 15's is depending on how serious your going with your build some aftermarket HD tie rods and steering components require larger wheels because their mounting locations are so close tho the wheel when mounted

 

This is why I went with 17" wheels when I bought mine. I will say that I've seen some 15" installed on a JK and the gap between them and the brake calipers was really tight. I can see how you might have issues with mud caking in the brakes.

 

This is the main reason why I went with 16" rims. When I was checking into 15's, I read about how with the 2012, the brakes are larger so there are not that many choices for 15's. Some will fit, but SO tight that you would have to shave the brakes a little to fit. Plus read that even if they do fit, it's tight enough to where rocks and stuff can easily get trapped. Not as much of an issue with 2011 and earlier, but it's still a tight fit.

That was enough for me. If it's going to be that tight, go with 16" or larger.

 

I had no problems with mud or rocks on a 15" wheel and brakes on my 08.

 

I've had 15's on my previous 2 Jeeps and the only time I ever had this issue, everyone else on the trail regardless of wheel size did as well. Some mud is just gonna cake in more than others. The 15's aren't gonna be any worse than the others. They just make for cheaper tires, hence the salesman is telling you to go with a bigger wheel. They get more money out of you that way.