Should I get Ultra Sun or Ultra Moon???
Note that I haven't played a Pokemon since Emerald
@plibbity There are a LOT of things that have changed since Gen 3, most importantly the Physical/Special split, how EXP is calculated, and the Mega Evolutions/Z-Moves.
Ultra Sun/Moon is a great place to start, though. Especially if you skip original Sun/Moon; there's some things that were taken out but don't feel "missing" in the Ultra versions of the games, if you don't know they were there.
@MariBari are the only differences between the 2 games the Pokemon you can catch and the available legendary? Or are there bigger differences now?
Here are all of the version exclusives, in case you wanted to check: https://www.serebii.net/ultrasunultramoon/exclusives.shtml
There are other differences, though, albeit small ones. Ultra Moon is set 12 real-life hours after Ultra Sun, so any gameplay during the day is at night and vice versa. There's also a couple instances of boss fights having different Pokemon between the two games, too.
I'm pretty sure that's all the differences-- no major story changes beyond the legendary or what time of day it is.
@plibbity Wait, I forgot one! The games have exclusive color options for buying your character clothes.
Sun has red, yellow, orange, pink, beige, and brown. Moon has green, blue, navy blue, purple, and gray.
You can also fudge the system by just buying white clothing (available in both games) and dying it in Festival Plaza, which is the online interactions component of the game ala Secret Bases.