Nathan > Aidan
Aidan's a straightforward person, and a good person, and probably way better boyfriend material than Nathan. He doesn't seem like someone who would try to deceive him, not even with the best of intentions, which is an important quality. He's nice. Euphie likes him a lot, and Aidan hasn't done anything to give Nathan a reason to dislike him.
Remember that time when Aidan referred to Nathan as his friend? Nathan does. Nathan's bad at following up on that being-a-friend thing, but he hasn't forgotten.
Nathan kind of hopes that Aidan is as protective of Euphie as he seems. Since she won't protect herself from Nathan and Nathan isn't protection enough, they really need an invested third party who is willing to be a guard dog. (He is all for Aidan breaking every bone in his body if he hurts Euphie. Good plan. Stick to it.)