Willy Loman is "willfully" able to break free from reality and dream about a life in which the American Dream is fulfilled: Lots of money and an achieved bundle of earnings so that he can provide for everyone. Unfortunately, this translated into illusions of grandiosity that led to mild bouts of insanity and, eventually, to his death. In his dream, Willy wanted nobody to ever need for anything. He wanted to be the head of a family (though he was by no means a fantastic family man) but the ideal of the "providing, hard-working man" was ever-present in his psyche to the point of developing into some sort of obsession that, eventually, took his sanity away.
Biff is less unrealistic than Willy. He understands the dysfunctional nature of his family and he confronts it. He dreams of going West because the West is the Universal meme for freedom (hence, the saying "Go West!") and in a way his actual crisis is his wish to detach himself completely from that family which has nothing in common with him.
When Willy kills himself, he did it so that the family could cash in on his life insurance. This is the epitome of his dream: Somehow to provide something earned from his hard work, which is actually also the epitome of the "American Dream". He had to pay for it with his life, while Biff had to endure an identity crises to understand where he is to go next.