While both Biff and Happy resemble their father, Willy, it is Happy who stubbornly maintains his similarity to Willy in the end. We see this in Happy's speech at the funeral.
"I’m gonna show you and everybody else that Willy Loman did not die in vain. He had a good dream. It’s the only dream you can have—to come out number-one man."
Biff, at this point, has realized his limitations. He has seen himself for who and what he is. This is a big step for Biff and it is a step that Happy fails to make.
During the action of the play, Willy is seen encouraging both of his sons to maintain an exaggerated view of their personal qualities. Willy supports the illusion that Biff was an important figure at the sporting goods store at one time. Biff believes this fiction as well until his actually goes to the sporting goods store and talks to his old boss.
"And then he gave me one look and—I realized what a ridiculous lie my whole life has been. We’ve been talking in a dream for fifteen years. I was a shipping clerk."
Biff recalls this moment when he is talking to Happy in a restaurant. In this episode, Biff tries to explain to Happy then to Willy that he is now done with the illusion. He can see now that he is not "great". He is just a normal guy, with normal limitations. This, for Biff, is a liberation and a positive realization. Willy takes it as a chastisement and an act of spite.
Here we see the break. Biff chooses reality. Happy chooses, like Willy, to maintain an illusion, preferring to believe in his own (potential) greatness.