If this is your first dabble with Gen VII of Pokemon, get this version! Better Legendaries in-game, the real time clock is set to standard daytime, the bulk of the exclusives are superior (Alolan Vulpix, Houndour line, Passimian, Blacephalon, etc.). The downside is that if you like Blue, Green, Purple, and Grey, the clothing and Festival Plaza's exclusives in UltraMoon or Moon are a better choice. You can still get Midnight Lycanroc in the wild later on, if that's what you were on the fence about.
As for the game itself, the story is nearly the same as Sun/Moon until you get to post game. That's when a familiar face, as well as many others, pop in. The graphics push the limits of the classic 3DS, the system I still play on. The only thing that really slows it down is double battles. I prefer the music in these entries to original SM, especially the Ultra Recon Squad's battle theme.
My recommendation is to start getting familiar with the Festival Plaza asap. It's the best way to get Poke Balls, Waters, and other items for DIRT cheap. That way you can use Poke Dollars for TMs, clothes, and other expenses. Festival Plaza also allows you to get tons of character customization outfits for talking to other people on Wi-Fi. And all it takes is just to link to the Pokemon Global Link. You can do a simple Random Trade, or deposit a Pokemon to trade for ones you don't have, and then you get A LIST of other players to get shops and clothes from. I like to check daily because people will have entire outfits that are hard to get, and the currency in Festival Plaza is just answering text questions!
Poke Pelago is also excellent for getting gobs of Berries, Fossils, Gold Bottle Caps, evolutionary stones, and even EVing Pokemon! You just deposit them onto the island, and come back a day later. This is sooo convenient for us that have jobs where we can't play Pokemon all day long. It's a good way to get perfectly EVed Pokemon, and even raise their levels for doing nothing. You get Poke Pelago on the second island after battling the Fire Trial, which is about 2/5ths the way in to the main game.
Some new additions like Mantine Ride, battle poses, Ultra Space, and some BROKEN Z Stones are sure to entice people as well. Mantine Ride is the best way to get Move Tutors of any generation yet, as you get to play Tony Hawk on the back of manta while dodging Tentacruels in the surf. I average about 9 points per ride, which equates to quite a few really useful moves for just 4 minutes of the mini game. Things like Knock Off, Stealth Rocks, Gunk Shot, Heal Bell, and Liquidation are highly coveted moves not only in-game but also for online battling.
The only reason I gave this one star less is that it's almost a carbon copy of Sun. We did this 20 years ago with things like Yellow and Crystal, but the difference is that Sun and Moon are very small games. They're in a tiny area, and the main story is shorter than say Black/White. Also, I cannot stand SOS Battles, and in these games you are forced to endure that mechanic all through the generation. But if this is your first experience with Gen VII, it's an easy 5/5. Another great Pokemon entry and a gorgeous sendoff for the highly successful Nintendo 3DS.