i think u will be better connected with Guy A, since both of you are shy and freeze up, and once you get past that stage your relationship with him will be that much greater.
Usually the quiet ones are the most loyal ones, but have a hard time expressing it outwardly, you might have that problem as well, in which case you will understand more.

Guy B, already has a gf, and you would be a bitch to be taking someones boyfriend away, without considering his girlfriends feelings, and he probably only sees you as a friend unless you force him into a corner with sex, in which case he would obviously go for it and both you and him would be in difficult situations and a pile of mess to sort out.
Guy B might be outgoing, and a boy boy, (acts solid around his mates, do and talk some funny things) thats normal.