There are a bunch of threads on this subject.
My 18s ride harsher than my 20s. 20s have plenty of sidewall, so the ride is more about tire brand/model than size.
The 20s provide better handling on the highway and back roads. 18s give you more margin against curb rash and debris. Looks are subjective, but my OEM 20s look much better to me than the OEM 18s. Most will agreed but there is some difference in opinion.
Tire selection in 20s is pretty good right now, and the OEM goodyears aren't bad if you get them balanced properly. Tire selection in the OEM 18s size is not much better. OEM 18s are meh..
If you do a lot of pavement driving, you should be happier with 20s. If you do a lot of dirt road or logging road driving, you will be happier with 18s or 17s.
20s are going to cost more, worth it is subjective.