I vote 20's n 35's I have had this combo on a few different trucks and it looks really good. It still has enough side wall for the amount of wheeling you are going to do with a big rim. The cost difference shouldnt be a big factor since you are spending a fair amount anyway may as well go with the bigger ones, plus chicks dig big rims ha.

 

Looks over function. 18s will be better for off-roading, period.

 

I know what you are getting at but we are talking about average wheeling, I am assuming he isn't doing hardcovers trails. I have a jeep on 35s and 15" rims and it's perfect. If you are gonna wheel it hard than get 17's...there aren't many people that wheel these trucks enough that they need to air down and require a large side wall. Plus the side walls are stiff for either size so it won't matter too much. I assumed he was going for looks over function since he was asking 18 -20's