The difference in the way that Biff and Happy Loman treat their father, Willy Loman, in the play Death of a Salesman lies on the manner in which Willy treated each of his children as differently as he did, and on the way that each of the young men developed psychologically as a result of it.
Growing up, Willy Loman demonstrated a marked favoritism for his eldest son, Biff. A product of Willy's own ideas of popularity, Biff was molded to act, think, and make choices the way that Willy would have wanted him to. At age 17 Biff makes the discovery that his father was having an affair with a woman, deflating completely the former image of Willy that Biff had created. As a result, Biff goes through a process where he has to de-program the constructs that he had built about Willy. In the process, he realizes that his own life has been nothing but a lie; a product of Willy's own imagination.
I realized what a ridiculous lie my whole life has been.
Because of this, Biff feels that he has the right to confront Willy with what he has come to know as "the truth" of their lives. For this reason, the restaurant scene becomes the epicentre of the battle between Willy and Biff: just when Willy is about to instigate Biff to lie about how well Biff's life is going, Biff retorts back with many efforts to "wake" Willy out of his fantasies. Biff is the only one who has accepted the facts of the Loman family's dysfunctional nature.
Happy, on the other hand, has never had to grow up because he grow up as a mere shadow of Biff. Since Willy bestowed so much attention upon Biff, Happy had to be play the "lighter sidekick" to such an enmeshed pairp. For this reason, he always aimed to accommodate everybody's whims and emotions so that he could be as accepted as Biff was. Hence, Happy is merely the "pleaser"; the careless "happy-go-lucky" manchild who has never grown up because he has ever been put in the position to accept any responsible role within his family, except that of being "the little brother". Notice how Happy, like his name entails, tells nothing but "happy" lies all through the play in order to please people.
"Im getting married, pops"
"I'm losing weight"
""I sell champagne and I'd like you to try my brand. Bring her a champagne, Stanley "
"But, Mom (Willy) had a great time with us"
Denial is so rampant in the Loman family that, even during Willy's funeral, it is Biff the only member of the family who sees the real reason behind Willy's wasted life. Happy, in his usual manner claims that he will maintain his father's dream alive. Since Biff knows that Willy had "the wrong dreams" all along, he is the only one to speak the truth. Happy will always be the one who pretends that everything is alright.