1-It will probably be real-time combat; don't confuse it with God of War action, you'll probably pick an action (attack or a spell) and wait for your character to prepare to do that action.
2-There are 2 FFXIII, because SE felt that VS wasn't different enough to warrant a XIV name. Plus the two games are a part of SE's Fabula Nova Cristallis compilation (no indications have been made that the stories will be connected, as far as I know).
3-Every FF has a new story, you don't need to know what happened in the previous game. However, for the fans of the series there are easter eggs, such as Chocobos and Sid.
4-This is a matter of opinion, but from what I've seen Versus looks better. It just looks so much darker, more mature, emotional, and more epic. But, once again, this one's a matter of opinion.
