15" wheels are cheaper and offer better sidewall flex off-road; 16" gives better tire selection in modern MT/AT lines.
15s on an XJ: long the standard, with cheap steel options (Cragar Soft 8, Pro Comp 69, Method NV) and excellent sidewall (more flex when airing down for rocks). Many heritage off-road tires still come in 15" sizes. The downside is dwindling new-tire options — many premium MT/AT lines have moved to 16"+ only.
16s: better selection in modern KO2/KM3/Open Country/Wildpeak lines. Slightly less sidewall flex, but adequate for trails. Steel and alloy 16" XJ wheels are common across Pro Comp, Method, Vision, and Black Rhino lines.
17s: possible with appropriate backspacing, but generally not recommended — too little sidewall for off-road, and you're spending money on rotational mass that doesn't help anything.
Don't go bigger than 16" if you plan to crawl. Don't go smaller than 15" — stock disc brake calipers won't clear.
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.