its sidewall of tire....you want sidewall for offroading......to an extreme but still applies in off road in general is look at trophy trucks, rock crawlers, etc....17in rims
there is more options possibly for 18" rims and cost less......nowadays with big rims becoming popular for show you can find quite a few though......
more sidewall gives you options for airing up or down, reduced pinch flats, reduced scuffing on rims and or rim catching on rock, root, etc and bending of the rim ( either cuz no air volume or spokes being close to ground catching a rock, root, etc).....tire flexes more with more sidewall, also your tire is the very first shock absorber, more air volume is better....its all pluses with no minuses
i have 20s but prefer 18s....ok for now as i dont plan on thumping my truck quite yet
smaller rims give a much better ride as well in general........street wont matter can throw whatever on but ride will be harsher