Sephiroth is a wuss, Seifer got sliced across the face, beaten up over three times by Squall (And also by Giglamesh, techniclly.) And was still standing, he'd win.
Seppy is more powerful. Sure Seif has more endurance, but I still think Seppy will lick Seif's blood from his Muramasa. but whatever. So did Squall. Now, i think Squall might win against Seppy, seeing as how Seppy has a long, but thin blade and it might allow squall to slip in a stab or two.