Xigbar is really fast, but he has close to no strength. He does control space, but Zexion has illusions, which, IMO, is just as good. I think it would be a close battle, but Zexy would win, beause he has super strong magic power.
Well, they're both dead... so neither. But if they were alive, I really have no clue. I would say Zexion, because of his illusions, but Xigbar is pretty strong on the offensive, but I would say Zexion.