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The problem is they kind of fucked up the story with things like changing Lusamine's entire motivation without actually changing enough of her scenes to indicate that, and removing one of the bigger scenes between the player and Lillie. But you miss out on a bit of content with the originals and Guzma doesn't really have a satisfying character arc in the original.
So you figure you should just play both, right? Well that's fucking hell when you can't skip cutscenes that are barely changes in Ultimate. So Ultimate just ends up feeling like a slog.
Ultimate isn't quite the definitive version of the game, but at the same time playing both is overkill. Sun and Moon is trapped in a very awkward place.