The Dai Seirei arc's plot is
so that shows how important Ray is to the story that one whole arc revolves around his girl, which is something that not even Kenja no Mago did, as that novel only focuses on the MC and his main couple and didn't spend one whole arc about saving MC's best friend or such friend's girl from a cruel fate.
Therefore, in terms of story and plot, Maou Gakuin is better than Magi's Grandson, as it focuses on the other characters too instead of overly glorifying and focusing on the MC too much.