While Pokémon Ultra Sun and Pokémon Ultra Moon follow the same story as the previous games set in Alola, there is a lot more available here and on the whole the game is generally improved with it.
To be honest, even on the surface that appraisal is an inaccurate one. The inevitable and somewhat misplaced criticism that can be leveled at these new Ultra games is that they are just a rehash of Sun and Moon. Obviously, there are differences on the Legendary Pokémon you can encounter depending on the version you are playing, as well as other Pokémon too for that matter, but the sheer quantity of Pokémon to go up against and capture is a big step up. However, the biggest changes and additions happen more towards the end of the game, such as the ability to battle with loads of Legendary Pokémon and the inclusion of Team Rainbow Rocket, which is sort of like a nostalgic greatest hits of all the villains from previous games in the series. The Alola region is somewhat bigger and a bit darker now.
