The two brothers, Happy and Biff, are alike in that they are both deceitful, unfulfilled, selfish, and caught in a cycle of mediocrity. They are different in that Biff displayed greater potential, reached a lower low, and has an epiphany at the end of the play.
Happy and Biff have both been caught in a web of lies and deception since childhood. As Biff states, "We never told the truth for ten minutes in this house!" Both boys deceive their father about their prospects and lie to him about Biff's appointment with Bill Oliver. They are also selfish, which they display in many ways, but most obviously when they leave their father alone in the restaurant and go off with their dates. Despite their braggadocio, neither boy has achieved much in life. Biff says to his father, "I'm a dime a dozen, and so are you!" Certainly this also applies to Happy, who has a lower position in his company than he pretends to have.
Despite these similarities, Biff distinguishes himself from Happy. During his high school days, Biff had real talent as a football player and could have gone to college on a football scholarship. Happy never amassed a wall of trophies as Biff did. But Biff ended up sinking to greater depths than Happy, even spending three months in jail in Kansas City for stealing a suit. By the end of the play, Biff has learned to look honestly at himself and at his family. After stealing the pen from Oliver's office, he realizes, "... all I want is out there, waiting for me the minute I say I know who I am." By looking at himself and his prospects honestly, he is able to begin forging a new and better future. Happy, however, remains deluded even in the Requiem, remaining tied to his father's failed dreams, vowing, "I'm gonna win it for him."
Willy Loman's two sons are alike in many ways, but Biff stands out as a deeper character who changes as a result of what he goes through while Happy remains static.
There is little comparison between the two except that they are, indeed, brothers, products of Willy Loman's lost dream. On a literal level, Happy represents just that, happy, a shallow, fleeting emotion. Even at the end of the play, he still believes in Willy's dream; "I'm going to beat this racket" (Requiem). Biff has tried to be what his father wanted him to be - the successful salesman, yet he realizes the emptiness of the dream and wants to leave the city to be in touch with something real, the land. You could say that both sons are symbolic of the two sides of their father's, Willy's, personality. Happy is Willy the salesman, "riding on a smile and a shoeshine." Biff is the inner Willy, the one who realizes the dream is empty; a man has to create or make something with his own two hands to be successful.