Actually, the point is Sion can take the first hit like a man and stab Bane in a moment of surprise. Bane doesn't have a likewise advantage. And I really see nothing to show that Bane was really much better in saber combat.
I like how everyone tries to find loopholes in Sion's invulnerability. Unless Bane chucks him into a star, the guy will survive the intial onslaught and defeat him.