I'm gonna have to go with Person B.
As ideal and dreamy as Person A would be, there's so much at stake for them that it feels one-sided. They're willing to do everything for me and lose everything for me, but would I be able to match that intensity? Would the MC be able to match that intensity?
With Person B, the stakes feel lower. Sure, they're a flirt, but they're not a cheater. They're loyal to the people they love. They put up walls, but what person doesn't put up walls? It feels more human, a bit more flawed, and you put in more effort to show personality (goofy, reliable, flirty).
That and let's face it, Person B has their boundaries whereas Person A sounds like someone the MC could take advantage of.