The biggest improvement in Battlefront II is that there's much more to the game than just multiplayer. There's Galactic Conquest mode, a single-player strategy/board game that lets you move around on a galactic map and fight battles on various planets. This is neat, but it's a throwaway extra compared to Rise of the Empire, an unexpectedly poignant story mode that unfolds from the perspective of a clone trooper. Taking players from the Battle of Geonosis to the Empire's last stand at Endor, this mode features far more complex missions than the multiplayer game and stands on its own as a great action experience.
The differences between each version of the game are largely cosmetic, although the PS2 supports fewer players than the Xbox version, which in turn supports fewer than the PC.
Star Wars Battlefront II is a fun romp that stays interesting no matter how you play it. It has its share of flaws, but for fans, this is the first Star Wars game to have it all.