17 or 18 size wheels
 

Tires are typically cheaper for 17" wheels than 18" wheels, but it's entirely your choice.

Either will work fine. 17's are more common. Overall there may be better selection of tires in 17" sizes. As noted, tires for 18's might be a bit more expensive. People bring up sidewall height when comparing cornering vs offroad performance, but I doubt you would notice a 1" difference between them. I didn't, anyway.

One thing to check is whether there are specific differences in the tire itself. Something like: the 17" version is C load range but the 18" is E load range.

I feel like there is a better selection of tires in both 17 and 20 inch sizes compared to 18's.

It's not an option, but I always likes 15 inch wheels and 35's on my old Chevy's, I like the look of a lot of sidewall, so I would go with 17's on the jeep

Another vote for 17s due to preferred look of the 17/35 combo. If you ever want or need to sell em in the future, there will be more demand for the 17 vs. 18.