If you haven’t been following Shin Megami Tensei: Persona 3 Portable this video will get you up to speed with most of the game’s new features.
The key difference is you can play as a female protagonist, which opens paths to new social links and events. Some scenes may even cross into Persona 4… Speaking of Persona 4, Persona 3 Portable borrows features from it such as part time jobs and player control over allies during battles. That should make Shin Megami Tensei: Persona 3 Portable easier,
Yes, this is another version of Shin Megami Tensei: Persona 3, but, as Atlus says, Persona 3 Portable is maximum Persona 3.