Golden does more to "fill out" things the game already had: it gives more backstory on the final boss and their goals, it adds to the social events that were already present, and adds versatility mechanics to both the party and the main character's Persona builds. The flipside is that certain elements feel a bit shoehorned in, and that can result in new elements being a little eye-rolling for some people.
Royal's additions in a lot of ways feel like a "sequel", but with elements integrated into the early plot so that plot threads later on don't just awkwardly come out of nowhere. This allows it to have a far more fitting and involved ending than P5 originally had, without technically losing the one that was already present, with some really developing characters and development even for Joker (something that's hard to do for a silent protagonist). However, it doesn't expand the pre-existing content much, so most of the original flaws are still there.
Having had a chance to finish and sorta genuflect on both, I'd say I suspect that Golden adds more, but Royal feels like more of an "upgrade".