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If all that matters to you is the story, SM is a better one. Especially for if the story is being told to children, I think the narrative actually tries to say something important. Tons of other media does it better, but it's not the worst attempt.
USUM is my go-to though. I'm a sucker for alien shit, I love the blue-skinned people and Necrozma's stuff to death, I can handle the butchering of Lillie's family's story if it means more sci-fi stuff. Plus, I think having all those extra side-quests makes the world feel more full, like I love the school mysteries and Left Poké Ball quest. And then gameplay wise, it's just no contest that the Necrozma level spike and new Totems just make for a harder and more engaging game.