P4 is absolutely better than P3 purely from a gameplay standpoint.
P3 is way more grindey, with long stretches of dead months where nothing of note is happening. The battle system has more depth than P4s, but it's less fun/engaging due to the existence of A.I partners (A.I partners are trash, debate me). Tartarus flips flops from tolerable to outright terrible. The game hits you hard with those slink requirements, so at times it feels like you're playing a dating sim (rather than a life sim). What do you mean you want to eat burgers after school to replicate an authentic high school experience? Lmao, hit the arcade or miss out starting Mitsuru's slink at a reasonable time, scrub. Slinks are generally worse. It has some of the best slinks in the series, but it's also weighed down by stupid shit like Kenji's social link, and it's incredibly obvious when the "we don't have friends that are girls" fact shines through. (They clearly get better by P4/P3P though). Finally, the the full moon bosses were uniformly terrible and it has one of the worst final bosses in RPGing.
All in all, it's much harder to outright replay P3 since it's somewhat clunky of a game. P4 by comparison just hits this sexy RPG sweetspot, making it infinitely more replayable. Like, there's a reason why it's considered a classic.