If I had to choose, SM would be my pick. Something about the USUM Pokémon models just don't feel right. The models are just like unpolished and bright. USUM does have more Pokémon though. I liked the new Ultra Beasts, but the story is just really bland.
I really liked a lot of things that USUM brought to the games (more interacting with Rotom, more Pokémon, etc...) but I have to say, I preferred SM. The story just felt so much more focused. USUM had some interesting story threads with the Ultra Recon Squad and Team Rainbow Rocket, but they just felt kind of disconnected from the rest of the game. The buildup toward things like Lusamine's true motivation and Lillie's backstory was the same as in the previous games, but the payoff was so jarringly different that it felt unsatisfying. That's not to say I inherently dislike USUM. Things like all those optional sidequests really fleshed out the region and Guzma getting a more complete story arc were fantastic, and I really felt like a lot was improved upon. It's just a pity that the presentation felt a little clumsy. I think the games would have worked a lot better if they'd either changed it completely, or only changed some details and made both just one game.
While I loved Sun and Moon's story with Lusamine, USUM felt more complete and had a much more satisfying ending. I was happier by the end; more so than with SuMo. The Team Rainbow Rocket post-game arc also gave me a lot to think about and spurred some interesting theories on Giovanni. Plus we have more options to get shinies which makes me happy as a collector. So...USUM for sure.
Overall, I thought Ultra Sun/Ultra Moon did better with an overall narrative, but it made me wonder why they even put out Sun and Moon when they clearly had a better product with the ultras. They added so much extra stuff to the Ultra games that really didn't add anything significant to the SM games.
I'm conflicted. All the arguments for each side are very good and compelling. I didn't exactly love the story in either games, but SM's story did seem superior in terms of continuity, but USUM, had better hype for the final boss and better closure. USUM also had better post game content where SM felt a little dry at the end. The Lillie was handled better in SM where, Gladion was handled better in USUM. If I had to ignore my personal enjoyment, I'd say that USUM was the superior game offering more to the player and a story that was only minorly inferior in some areas and minorly superior in others. Looking back at other games, I'd say this isn't an inferior remake. Platinum and emerald did make a lot of changes to its original source material, but for the first two thirds of the games, it was practically a carbon copy of the last two games, just like USUM. Only B2W2 mixed up the formula simply because they screwed up the ending of BW's and they needed to make up for the lack of conclusion. I think the 2 year jump they decided to go with was for the better, but they seemed to take a set back with USUM giving what I would have expected from a BW remake, just fix the ending rather than rewrite the story. Now if USUM started on one of the smaller islands in Alola that we never see in game after the events of SU, then I don't think we'd be having these conversations.
I liked the story more for Sun and Moon. It’s more cohesive than USUM’s because of Lusamine’s motivations. And I agree with what others have said about how Lillie had better development. My main problem with USUM’s story, however, is that not enough was changed to rightfully call it an alternate universe. Most of the story changes, in fact, can be summarized by the presence of the Ultra Recon Squad. Too much of the main story was copy-pasted word for word - even the Aether Paradise scene with Nihilego was the exact same when it made absolutely zero sense for Lusamine to act that way with her USUM motivations. But I will give USUM credit where it’s due. I found the totem battles - especially the ones on Ula’Ula Island - to be significantly more challenging this time around. The user interface was clean and everything you could want in a Pokémon game - there was even a shortcut button to save, which you don’t realize you love until you open up an older game. They also added a lot more flair to the region itself, specifically with the side quests. Those provided so much world building and all-around cute or fun moments. I’m still personally torn on which game I like better. Do I value the story more, or do the many little improvements in USUM win me over?
Everyone who starts playing Pokémon can skip SM, because USUM is like a updated Version. There are some differences like the Totem Pokémon, but USUM surely is the better version.
I prefer USUM over SM, but it is a shame that USUM wasn't THE gen 7 release and it lost a lot of momentum due to the fact that the story wasn't different enough from SM to really make themselves stand out.
I haven’t had a chance to play USUM yet, but it’s my understanding that there was better variety in Pokémon. I still prefer SM (not just because it’s the one I played) because I think the story line is better than it is in USUM which I think is an alternate timeline of SM.