Biff and Happy are similar as the play opens. Their differences become clear and also grow as the play goes on. However, from the start, Biff is the brother who owns the greater affection and esteem of Willy and Linda. Willy would love to have Biff find success, especially if that success resembles the dream that Willy has dedicated himself to pursuing.
Both brothers do initially follow Willy's example, considering themselves better than others, believing this fantasy and hoping to achieve true greatness through strength of personality.
Biff and Happy share their father's tendency to concoct grand schemes for themselves and think of themselves as superior to others without any real evidence that the schemes will work or that they are, indeed, superior. (eNotes)
Biff manages to escape this illusion, while Happy remains moored to the dream. After meeting with his former boss, who does not remember him, Biff realizes that he had been building himself up with illusions. He had never been a salesman at the sporting goods store. He had only been a shipping clerk.
I even believed myself that I’d been a salesman for him! 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. (Death of a Salesman)
From this moment on in the play, Biff tries to convince Willy that the false dreams they both have been following are unnecessary. While he makes this change, Happy remains convinced that he is special.
He is clearly over-compensating for insufficiencies of character and of self-esteem, and trying to gain what affection he can from Willy and Linda. Happy has always been the less favored son and he knows it.
One way to state the difference between Biff and Happy at the end of the play is to say that Biff has broken away from the path defined by Willy, while Biff remains dedicated to following that path.