>>56977418I'd argue it isn't worth playing both. Or at least not-back to back. And my argument is that, as a self-contained story, SM are the way to go.
But if you want something to do afterwards, it's lacking in post-game.
USUM and SM both have the battle tree, which is your 'battle gauntlet' facility. USUM has your bare-bones Tony Hawk Pro Mantine Surfing mini-game. And Ultra Wormhole exploration, which increases shiny chances for a select few pokemon, and the ability to find legendaries and more Ultra Beasts. USUM also has a photo studio where you can pose your character, and take pictures of you and your pokemon.
I'd really recommend SM. But if you never play SM, you won't get annoyed at the changes they made, because you won't experience them.
So USUM is acceptable.
Again, it really depends on how much stock you put on story and characters. Which, I argue is probably the thing Gen 7 does best, so why not see it at its best? But USUM will give you more to do.
