Final Fantasy Versus XIII

So, I've just played (Final Fantasy-related games) Chocobo Racing, and rented Final Fantasy XIII some time ago. Until now, FF XIII seems pretty awesome to me, and I would like to buy it, but I feel it wouldn't be a good decision, since I would only have some part of the story remaining and would be better to rent it another time if I don't finish it, also I would like to buy Assassin's Creed Brotherhood and Little Big Planet 2, and I don't have enough money. So I would rather like to buy Final Fantasy Versus XIII when it's released (Yes, I know it won't be soon, but I still would like to buy it).
So here's the important part:
What will be the differences between Final Fantasy Versus XIII and the actual XIII game? (I know it will have a different story and all, but I mean about the gameplay).

Also, some Final Fantasy games I should play?