16" tires are generally cheaper than 17's. Only problem I've seen with a 16" tire though is that most of them seem to be a D or E load range, which is great for durability and handling, but it makes your ride much harsher. I'd say go 15" or 17". 15's will fit if you get the right backspacing and the right wheel. There was a thread on here somewhere listing confirmed wheel fittments.
SOLD-08 Unlimited X, "jeep green", auto, limited slip, hushpower muffler, Aerotech Interceptor gauge,Powerstroke tranny cooler, skidrow evap skid, AFE Pro 7 filter, Teraflex leveling kit, EVO control arm skids, 15x8 Black Rock 909's, 33" KM2's