Dante would win to be honest. I know that Sephiroth hasn't ever exerted his full power, thus, he's a lot more powerful than he appears to be in battle, yet for some reason, he never goes full force, and because of it, has lost to Cloud twice in his most powerful incarnations. However, Dante has conquered the highest demons of hell as well as the Emperor himself and he only ever seems to get more powerful till DMC2 where he could be practically called invincible for real. Dante can brush off hits that would normally kill a man, having been impaled by his own sword of choice three times in a row and the fact that he has become so powerful on his own without external forces (Devil Arms don't count as the selection is inconsistent) proves his worth. Sephiroth on the other hand, obtained his power by absorbing Jenova, his supposed mother.