USUM had a much wider variety of Pokémon to catch, and I do like that Rotom actually speaks to you more often (although it became a little too often). The new Pokémon are really cool. I love the look of Ultra Necrozma. That was one thing that was, fortunately, not spoiled for me so I saw it for the first time in-game and was just blown away by it. Story-wise though, I'd have to say I prefer SM. Lusamine's motivation just changed too drastically. She was a lot more of a villain in SM, which I liked. It felt like Lillie and Gladion really had a reason to have wanted to leave. I like that USUM focuses SLIGHTLY more on Gladion, but it isn't done very well, and there are some good moments with Lillie in SM that just never happen in in USUM. And then there's the post-credits events. USUM's was way too short for how much it was being hyped up, and just wasn't nearly as fun or story-oriented as the post-credits event from SM. So over all, I prefer the general gameplay/Pokémon availability of USUM, but I greatly prefer the story of SM.