Difference between Pokemon Sun / Moon and Ultra Moon / Sun and which is better?

So I will get myself a new Pokemon game for the nintendo 3ds soon, but what is the difference between the sun / moon and ultra sun and moon parts? So if you have the normal sun and moon edition, is it worth it to get the ultra edition? Is there another story and everything?

If so, and the ultra editions are also cool, different and such, which of the two would be cooler ultra moon or ultra sun?

One could more or less describe the sun / moon as a very detailed demo version of ultra sun / ultra moon. In terms of the story, there are only slight differences like other ruler Pokémon and that the Ultra Dimension and Necrozma are more involved.

Ultra sun or ultra moon are equivalent for me, so I would rather go after which https://www.pokewiki.de/Editionsspezifische_Pok%C3%A9mon#Zusatzedition_5 Pokémon you would rather have.

The basic structure is the same.

You get the same starter Pokemon, many Pokemon have remained the same, have a similar story and many characters have remained.

At the same time, there are 100 more Pokemon in USUM. That's quite a difference.
The story differs in many ways. There's now the ultra-research team that they will come across again and again, dialogues have been revised, changed and USUM just feels like a mixture of special edition (i.e. What used to be yellow, crystal, emerald and platinum) and revised game.

The story is more about Necrozma, the old plots no longer exist, characters are now in focus in a different, much better way.

Plus there are more areas to do more, more opportunities.

Mantax surfing is quite enjoyable and finally offers a humane way to farm GP.
Each ultra beast has its own ultra dimension that can be visited multiple times.

In general, significantly more ultra dimensions have been added, with those without ultra beasts being kept simpler and actually only leading from a path to a portal with a Pokemon.

There the chances of a Shiny are greatly increased, depending on how well you did in the previous mini-game.

Legendary Pokemon can appear there too, but at regular shiny rates.

Some rulers' Pokemon have been changed, and a few tests have even been completely redesigned.

Matsurika finally has an exam (in SM she just gave you the Z-crystal and then left).

Overall, many places have been revised again and presented in more detail. And that, although it was actually said that with SM one would have exhausted the possibilities of the 3DS and the game modules to the maximum.

USUM are de facto simply the better, but by no means the same games. The experience is completely different. Admittedly, the beginning is pretty similar, but that changes after the first exam at the latest.

Since you have already been told in the other answers that you should buy one of the ultra games, I would just like to explain the differences between (ultra) sun and (ultra) moon. On the one hand, there are of course exclusive Pokémon. Which you find better is a matter of taste. But also the time in the moon games is shifted by 12 hours so that it is night in the game while it is day in the real world. To be honest, I think that's a stupid idea, because it darkens the screen during the day and then it's too bright at night.

Conclusion: buy ultra sun!