If you have played P3 FES, you have played the best version, though the only main differences of this is some extra dugneon I believe, female character and different character links for her, this also lacks the 3dmodels on exploration while in town, not that it ruins the experience from what ive heard from friends that imported it, and actually makes it less tedious after a while when moving your character around while on day, also this one lacks the P3 FES content.
I in my case, never got the chance to play P3 so this is a perfect chance to catch up, since I hate how upscaled PS2 games look on HDTV, never could get used to it.
I own ALL the games in this series so far..Obviously can't have the PSP version yet But, I am getting it to add to my collection. PLus I just ordered up myself a PSP until now I've been "borrowing" my 18 year old sons. I have found MANY games that I might not have seen or heard about if not for the PSP. I say if you can afford it then why not?
There aren't any differences to the overall ending or the main events of the story, but you can change what happens to a certain character depending on how far you get into his/her social link. You can also change the ending slightly during a New Game+ playthrough, though it's nothing major.
The main reason you'd want to buy this (and the main reason I'm getting it) apart from the social link changes would be the revamped battle system, since you can control your party members directly in battle and decide their skills a la Persona 4.