Well Domination and Supremacy are very similar, Supremacy is being above everyone while Domination is controlling everything around you ( Atleast that's what the dictionary says) so i'm thinking Issei's going to come out on top not because he's the king but because of the trait of Domination
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Now about Saji vs Issei and the match as a whole Issei's going to win no doubt about it but i'm thinking this fight is going to be the trigger for a lot of "problems" on both sides.