Sion is a little too full of himself; it's like he sees Mia as someone who measures up to him instead of a human who could be flawed in ways he doesn't like. Abel, on the other hand, knows his flaws and now has the motivation to work on them. Between the two of them, Abel is coming out ahead in terms of viability as a romantic partner.
Then again, neither of them has the same inevitability as Guillotine-kun.