I think he's much better than Malak.
His voice is TONS better, he has a better face and skin, has a totally better personality, and could NEVER be redeemed like Malak could.
Also, I REALLY didn't like Malak. His voice was annoying, *SO WAS HIS LAUGH*, and and faceplate was a real turnoff (not in g*y terms).
What I really like about Sion is that he is the definition of a True Sith.
He is completely cold as ruthless, stops at nothing to achieve his goals, and can never be redeemed, only defeated. I think inability for Sion to be redeemed makes him a badass.