No idea if it's the same demo as what was presented at E3, but I can assure you this much: the gameplay is virtually the same from the first game. If you hated the Synergy Combat Battle System, then you'll hate it here as well. There are some subtle differences, like the ability to capture enemies a la Pokemon-style, but it won't be enough to change your feelings on the combat system one way or another.
So if you just want to see how pretty the game looks or how Serah performs in battle—she was crystallized in the last game, afterall—then you will enjoy what the demo has to offer. Since the demo will not really offer much in terms of the story, I'm going to sit it out and just wait the extra 21 days.