If one doesn't have Sun and Moon? I cannot reccomend it enough, for it is a great time, and I'm sure anyone who missed out on the first run-through is gonna have a blast! I'm loving the story and the characters so far.
But you have the original Sun. And while I never played the originals, there are some who have who feel it isn't worth it. But I also have a friend who thinks it is worth it. So in that case, it's a bit trickier. I can only say it's all up to you.
If one doesn't like updated versions of existing games and has the originals, it's best to hang on to them. If the new stuff is at all interesting to you, you can snag it, but I personally suggest holding off on actually playing it if you played the originals, at least until you feel like revisiting Alola's story. You'll have to go through the parts you already went through to get to most of the new stuff, so you're not gonna have a good time if revisiting the story is something you're just not ready for.
I really don't play for the competitive scene, so that, I can't really say. I would suggest looking to those who know about it for info on that. They'd know better than I do, at least.
So basically, it's all down to you. To people like me without the originals, this is a fantastic game that deserves to be played, so I can't reccomend it enough, but to someone like you who does, I don't wanna say "don't get it," but it's up to you if the changes and addtions are worth the additional $40.