Although I agree that Sparda>Majin (at least for now) the fact that Dante is half demon while Sparda is a full one doesn't prove anything.
Dante=Half Devil
Random Bloodgoyle=Full Devil
Berial=Full Devil
And yet they are no match for Dante. Let's not forget that Dante's power is going higher and higher in every game, so the probability of him surpassing Sparda at some point certainly exists.
(PS: I really hope they made that Sparda game that we all wish to. It would give us answers to SOO many things. What made Sparda rebel against the Devils?Why was he the most powerful Devil and yet he was not the Emperor of the Devil Kingdom? How did he start the war against Mundus and his minions?What kinds of powers he possessed except the ones we've already seen by Dante?And stuff like those. But Capcom doesn't seem very eager to go at it. Although I can understand why, but anyway..)