Ultra Sun and Ultra Moon are mostly the same, but there are still a few differences. For one, in Ultra Sun you can get Dusk-Mane Necrozma, while in Ultra Moon you can get Dawn-Wings Necrozma. If you evolve Cosmoem in Ultra Sun, you will get Solgaleo, while in Ultra Moon it will evolve into Lunala. If you evolve Rockruff in Ultra Sun, it will evolve into Lycanroc Midday form, while if you evolve it in Ultra Moon, you will get Lycanroc Midnight form. You can see all the other version exclusive Pokemon here. Of course, there's still the same time difference between Ultra Sun and Ultra Moon as there was with Sun and Moon. When it's daytime where you live, in Ultra Sun it will be day and in Ultra Moon it will be night. When it's nighttime where you live, the opposite is true. Also, as with Sun and Moon, in Ultra Sun you will be able to purchase clothes with brighter colors such as red, orange, and pink. In Ultra Moon, you will be able to purchase clothes with darker colors such as blue, purple, and gray.
In conclusion, not much is different between Ultra Sun and Ultra Moon except for the version exclusives. However, I still think you should try playing Ultra Sun because it is a different experience than playing Ultra Moon. You can find different Pokemon and set up a different team.
I got most of these answers from here.
I hope this helped :)