One of the chief differences between these two characters lies in the way that they respond to their father's death. Happy seems likely to reproduce the characteristics that his father displayed. He insists that he is going to show that Willy Loman did not die in vain and that he is going to "win it" for him. Yet competitiveness has already been shown in the play to be ultimately fruitless though it sustains the capitalist system. Biff's claim that his father "never knew who he was" and that he himself does suggests that he will move away from his father's model for success.
This itself raises another crucial difference between the two brothers. At the beginning of the play, both are described as "lost", but it is Happy who oozes confidence and refuses to give in throughout. Biff, on the other hand, changes from a "lost" boy to a man of some insight and responsibility. Notice how Biff and Happy clash about revealing the truth about Bill Oliver. It is clear that Biff is sensitive and caring and loves his family deeply, but in the end the kindest thing he can do is to be cruel and force everyone to face the truth. This is why he reveals that fact the he has been in prison for debt.
Although, by the end of the play, it is clear that Biff does not precisely offer a counterbalance against the imaginary excesses of his father, in his protestations that he is an ordinary man with no pretensions at the end of the play he seems to be identified as the character who has the clearest understanding of what has gone wrong in the family, and, in this sense, he perhaps represents some hope for the future.