If you haven’t played Persona 5 yet, go with Persona 5 Royal.
Persona 5 Royal is basically just Persona 5, but with extra content. It’s not like a game of the year edition though; the extra story and character content you get in Persona 5 Royal is simply unavailable to Persona 5 players.
Additionally, all the DLC that you bought for Persona 5 needs to be bought again for Persona 5 Royal, since they didn’t include that in the game. Though, if they do the same thing overseas as they did for the Japanese release, it may be good to be an early adopter, as for the Japanese release they dropped the price of all DLC not exclusively made for Royal by something in the realm of 95% (I was paying pennies for all the content).
There are a number of quality of life improvements that certainly make the whole game a bit better to play. Things like feedback in certain places, some slight improvement and added variation in level design, and a certain procedural generated dungeon that has a bunch of things added to make it less of a drag, and much more.
Plus, the content and endings from Persona 5 are still there as well (or at least, I think they are) so you would not be missing out on any ending either as far as I know.
Even if you disregard the quality of the added content, the quality of life improvements are pretty significant and numerous and those alone would make it much better to buy Royal over the vanilla version of the game.
If you have played Persona 5 already on the other hand, there is not much reason to return to Royal unless you are a big fan of the original. I realize this is not what the question asked, but I felt it important to add.
Personally, I think the added character and dungeon content is not all that fantastic. The new character you are supposed to get will only join you very late in the game, when you are basically already armed to the teeth and completely set in your patterns. By then the new character will be just a bench warmer.
The extra dungeon is not that bad (even if it does get a bit tedious later on), but I don’t like the narrative surrounding it as it kinda ignores the narrative rules Persona 5 set for itself at the beginning of the game (and no, Mr. Game, having your characters acknowledge this and then do nothing with it is not sufficient explanation).
I like the new ending though, unlike many of the fans. I like how it’s a bit bittersweet.
edit: A small edit, since the game is out now and I know how they handled the DLC. As it turns out, overseas customers are in luck! All of the Persona 5 DLC seems to be downloadable for free in Persona 5 The Royal as the “legacy” bundle. Just wanted to clear that up, as in my initial answer I based my answer on how Atlus tackled the Japanese release of the game (who didn’t get a legacy bundle).