The main difference between the characters of Biff and Happy Loman is that, in the end, Biff gets to realize that his life has been a lie. Although Happy is aware of the same situation, he continues to follow his father's steps and keeps denying that anything is wrong with the Loman family. However, Biff explains the situation best when he says:
"I realized what a ridiculous lie my whole life has been."
As a result of this discovery, Biff decides to change the vicious cycle that the Lomans live of denying reality, confronts his father about it, and chooses to leave the Loman household for good.
Meanwhile, Happy gives due justice to his nickname. He is the "OK" man. Everything is fine with him, even if its under denial. He tries to dissuade Biff each time Biff discovers that something is just not right in the way the Lomans act. That is, perhaps, because he is used to living under the shadow of Biff and does not know how else to proceed in life. Eventually, however, he falls under the Loman spell and tries to follow Willy's sales dream. Therefore, Happy perpetuates the cycle that Biff is trying to eliminate.
Yet, prior to Biff's epiphany the brothers were inseparable in their combined co-dependence of each other. They were clueless, immature, enmeshed, and unable to act like responsible adults. They were both psychologically castrated by the overshadowing past of Willy's control over them. They used to be lost children, basically, until finally Biff grows up and cuts loose. This is how they are very different, but used to be very much alike at one point.