Ok, sit back and relax because things get complicated.
Persona 3 Portable was the third version of Persona 3 and the only one on PSP, meaning in order to get it to work on a handheld device, they needed to cut out the anime cutscenes and limit town navigation to a menu, like a visual novel. While in Tartarus however, you still control your character and you see everyone on the battlefield, so only outside the dungeon crawling does it look super-scaled down. Also, the epilogue that was added to Persona 3 in the FES (second) version, The Answer, was cut completely. So, this sounds super inferior, right? Well, the main selling point of Portable was the ability to play as a girl (FeMC), which added in brand new social links with male party members (and also new NPCs), replaced some music with new tracks, and changed the UI color. The FeMC also has different dialogue choices in the story, giving her a very different personality from the original, male MC. Her inclusion has been very popular with fans (myself included) and she has only appeared in Portable. No other versions of Persona 3 have her. And of course, the male MC is still in the game, so you can choose male or female at the start.
Persona 3 Reload is a ground-up remake of the original Persona 3, with some changes based on FES (the ability to directly control your party members and the Aigis social link). Reload is considered the definitive version of the original Persona 3, and I emphasize that because there is no FeMC and no The Answer. The Answer is being added in September through the Expansion Pass, but if you just buy Reload base, The Answer will not be there. So, if you want everything Reload has to offer, you need to get both. Reload also has a brand new English voice cast, but they all do a good job and if you have no nostalgia for the original cast, this change won't matter to you. Other than that, the game looks beautiful and is a great way to experience the series for the first time. Yes, FeMC missing is a blow, but I understand including her probably would have added a lot more time to development. Cutting The Answer just to sell back to you later, however, is really damn shitty. Fatlus gonna Fatlus.
So, to summarize: Portable has inferior graphics and no The Answer, but it's the only way you can play the FeMC route. Reload is hands-down the definitive version of the original, male MC story, but it's also missing The Answer unless you buy it separately and FeMC is gone and most likely won't ever be added. In my opinion, if you've never played Persona 3 before, go with Reload unless you desperately want to play as a girl. Yes, Portable has both gender options, but the new additions to Reload make it just a fantastic JRPG. Plus, playing the male MC before FeMC is highly recommended so you can appreciate the changes to the narrative. The only drawback is that if you do want to play Portable after Reload, it'll probably be pretty tough to go back in time to a much more dated game.
Hope that helped!
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