Should I play SM or USUM?
USUM is mostly regarded as a better version of SM. With a few new changes, in cosmetic, improvements to quality of life, better post game and what is to me a better sense of closure to some of the central plots. You are better off just getting USUM, due to most of the game being the same as SM.
Should I get ultra sun if I have moon?
Overall, it comes down to this; should you buy Pokémon Ultra Sun and Pokémon Ultra Moon if you already bought and played through Pokémon Sun and Pokémon Moon. The answer is an emphatic yes.
Is Ultra Sun harder than Sun?
They are an expanded version of regular Sun and Moon, so the Ultra games have more content. The Ultra games are only very slightly more difficult, but Pokemon games are never that hard anyway.