15" rims are better off road due to more sidewall and less chance of rolling a bead while aired down. tires are also cheaper for 15" wheels which is nice. stick w/ an 8" wide wheel, you'll have less rubbing and wont wear stuff out as fast. 31x10.5 is about the biggest you can fit stock, i still rub 32x11.5's w/ 2.5" lift.