I had 18s and 17s on my Jeep. I took both off-road and there is no real world difference. My new TRD Off-road Premium came with 17s. I'll be keeping the stock rims cuz I don't care about changing the looks. 17x7.5 works fine for me and I don't have a reason to change.
There is no real world difference is a 33x10.5 in 17in vs 18in. The outside diameter and width are the same on both tire/rim combos. There is about 1/2in of side wall difference but thats it. The side wall difference means nothing.
In general there are more tire options for 17in vs 18in.
In the end its all personal preference. 17 vs 18 will have no real world difference.