I wonder if part of the reason that Seishin isn't so (outwardly, maybe inwardly) swayed by Toshio's words is because he already knew these things about himself all along; hearing them from someone else turned them into fact rather than speculation. Don't you think that Seishin benefits from these deep discussions with other people? Sunako cracked his personality pretty well, and he's more comfortable with himself after this. Now Toshio has as well, and that leads him to be more comfortable with his decision to join the Kirishikis, probably.

Yeah, it would be a mistake to say that Toshio is the better character just because he's more outwardly impressive, so to speak. They're both very complex characters who choose different ways to live their lives and view the world differently; that alone doesn't make one of them a superior character to the other. It's also really original how rather than having the two foil characters be enemies, Ono made them best friends, and explored what kind of conflicts it would cause to have two such different personalities be around each other and have affection for each other.