Well purely for post game, USUM is definitely better than SM, with the whole Rainbow Rocket and Legendary hunting though I still think other third versions like Emerald, Platinum and BW2 (not a third game I know) had superior post game. It also adds move tutors which is always good.
Beyond that, the main game is almost the same, with all the changes occurring towards the end. And this is my opinion personally, but the USUM plot is actually inferior to SM. USUM is also harder than SM if you like challenges.
I personally think USUM > SM, but that doesn't mean much because in the end, I dislike both games, and hate gen VII as a whole. A lot of the original issues in SM are still there in USUM, such as Festival Plaza being crap, SOS battles as annoying and random as ever. Even the new feature Battle Agency had good promise, but is unfortunately terrible because it's almost impossible to find a high ranked trainer.
So, I would recommend getting USUM only because to keep up to date with all the Pokémon, and with Move Tutors and Legendaries, making it a more complete game. However, purely from a game experience, it's not worth it at all imo.