As of now, just Pokémon Ultra Sun is my choice (specifically because of Solgaleo being awesome(-r than Lunala), and the sun being fiery), provided that it is a game that sufficiently enhances Pokémon Sun in the vein that third versions usually do to their paired counterparts.. I originally wanted Pokémon Sun, but don't have it yet, so now I'm holding off for just Ultra Sun if it is outright superior. Otherwise, if there's a reason to get both Pokémon Sun/Moon and Pokémon Ultra Sun/Ultra Moon (similar to how you get an expanded storyline and a double dose of music if you get both Pokémon Black 1 and Pokémon Black 2), then I'll get Sun at some point and Ultra Moon later (to try the alternate story and experience the "opposite version" - unless Ultra Moon's "story" somehow more resembles Sun's story.. -.-).
I would get Pokémon Sun alongside Pokémon Ultra Sun only if you have to get the matching version for story purposes (like how if you got Pokémon Black 1, you would specifically have to get Pokémon Black 2 if you want to collect all of the Tao Dragon Pokémon in one game - namely, Reshiram, Zekrom, and Kyurem).