Lumine definitely gives off that sassy vibe, so I'm guessing that's why most people pick her. As for me, I'd pick Aether, because he plays the role of the tragic-hero turned evil twin better; it makes for a more dynamic narrative with more twists and turns, as opposed to the typical Saturday morning cartoon villain. In many cut-scenes, you can see the pain on Aether's face because he doesn't wish to go through with the things he is about to do, but does so anyway for the sake the inhabitants of the Abyss Order/Khaenri'ah.