Be very careful if you want to go with 15 inch wheels. The 4 cyl frontier has smaller brakes and can get by with a 15, but those 15 inch wheels will not fit on the 6 cyl trucks that have larger brake calipers. You may find a 15 that may fit with a wheel spacer, but I do not think there are any that fit directly on the 6 cyl trucks at this time.
I prefer more tire than rim with truck tires, so I am weighing my options. A lot of times you can go down one size. I am not sure which size I want just yet....just checking with you guys...maybe someone tried it.