Caesar wins, he can stop himself from breathing the poisonous gases, aka the only way for mallegan to hurt him. Few gastanets>>>shinokuni>>finally removes air from the area
Ceasar low diff, Magellan's strongest point is his poison, the fact that Ceasar is a logia and the fact that he's most likely immune to poisonous gases he takes this rather easily.
As I though, people would be split on this battle.
I personally see Caesar taking this, due to his control over the gas in the Atmosphere.
Though none of Caesar's poison would affect Magellan, his explosions would blast Magellan to Hell.