>>57224873
>first
Just play one.
Ultra is marginally better, but you aren't missing much by skipping it.
>T. played the original and Ultra at launch but haven't beaten Ultra due to tedium.

>>57224873
They didnt add much to Ultra sun/moon just like 3 new ultra beasts and the legendary fusions with Necrozma.

>>57224873
Ultra has more content and better story encounters but worse actual story. If you don't care too much about that, go with Ultra. If you do, probably go with Sun first because the Sun main villain is more interesting than in Ultra.

>>57224935
Heard Ultra added more difficult encounters. Does that apply to the main story too or is it only some endgame encounters?

>>57225607
USUM has a single cheesy gimmick boss during the main story and that's about it in terms of "difficult" encounters (not that anything in these games is difficult)
i would just play regular SM because they made the rotom dex super annoying in USUM for literally no reason and that's enough to make me not want to ever play the game again

Ultra is better long-term for difficulty, but both Alola games are really slow-paced due to having not just more but longer unskippable cutscenes than any game in the series besides maybe SV. The story in Ultra is also dogshit.
For your first time, play the originals for a decent story, then come back in a few months or even a year or two for Ultra so as to not burn out on the region and experience, by pure game play, one of the better campaigns.
>>57225607
Both of them are a bit harder than the average game, but Ultra is the only 3DS game balanced around a toggleable EXP share, gives specific IVs to certain trainers and trainer classes, and almost every major trainer battle has perfect IVs, battle-ready EVs specifically chosen per mon, and great team composition in spite of some random monster having only 3 moves at times. Guzma's Masquerain is especially hilarious.
Again, give SM a shot, the come back in a few months like the games did and give Ultra a shot later.