Technically, smaller rims would better for offroading with the right sized tires.

Pros: 33" tires on 16" rims would have more sidewall than 33" tires on an 18" rims. A bigger sidewall is better for offroading because there's more rubber that can be used in rocks/ruts without damaging the rim. Your 18s are more likely to get damaged when offroading because they ride closer to the ground. A lot of rock crawlers have 15" rims with massive tires.

Cons: Buying tires would be expensive if you are only going a few days out of the year, 18s are bling-y-er

I would say 18s can look nicer with big tires and will be just fine offroad as long as the sidewall is big enough. With 16" rims you could get a bit smaller tires and still have a decent sidewall. It's really depends on what you want. Either way, 16s are good for offroading.