I would argue that Willy is the play's central character. Though Biff has a flashback scene, it includes Willy. Willy is Biff's fixation and it is Willy's fate that this play is interested in exploring centrally, not Biff's.
Willy, simply, receives more attention in the play and is more fully explored emotionally, thematically, and narratively.
Willy is, no doubt, the play's main character; Biff, however, plays a particularly significant role in the play. Besides being a dynamic and well developed character in his own right, Biff is a functional character in the play. It is his discovery of Willy in the hotel room with the woman that originally poisoned his relationship with his father, and it is their conflict that largely drives the play and forms its subtext. Biff functions in another capacity, as well. He is the living proof of failure--Willy's failure as a father and the failure of the corrupt values upon which Willy built his life, his career, and his family. Happy's life echoes failure, as well, but Happy continues to skate through life, unaware of his own superficiality and unimportance. It is Biff who gains self-awareness, sees his life for what it has been--and suffers for the sins of his father.
Death of a Salesman is a modern tragedy. The definition of a tragedy is a story that tells the downfall of the main character as a result of his own mistake. Biff experiences hardship in the play - he certainly isn't doing well in his life - but Willy is the one who experiences a downfall. His illusions about life are slowly destroyed to the point that he commits suicide.
Consider how the action of the play is organized. The flashbacks are structured to demonstrate how Willy got to the stage he is at right now. We see him in contrast with his brother, showing how it is that Willy was set up to believe in the possibility of "get rich quick" schemes. We see how he lost face as a father when Biff discovers his infidelity. We see Willy face off against Charlie and admit that Charlie is his only friend. Biff's storyline of seeking for a path in life just highlights Willy's mistakes. We see in Biff, from the teenage Biff to the current one, a man who believes too hard in the power of name and appearance over hard work and on a man who believes in getting rich quick. Biff's conversations with Willy and Happy help to underline the tragedy of Willy's life. There is no moment of change for Biff.
Consider also that the other characters are concerned with and center around Willy. Linda protects him, Happy seeks his approval, and Biff tries to accept his flaws. Miller structures the story around the father in the family, not the son. The play doesn't end with Biff's new life - it ends with the family, and Charley, gathering for Willy.
I think it has to be Willy. Without him, there would be no play which is clearly about the inadequacy of his vision of the world in the time in which he lives. Biff is a product of this failed vision; Willy raises Biff to think that being well known, well liked is the key to "success." He fills Biff this half-truth, so much so that Biff is fails to do the other things you must do to be success ... such as passing algebra (although we know that his failing algebra has as much to do with discovering his Father with the "buyer" in Boston).
So I see Biff as an important "minor" character whose purpose is to enflesh the incorrect decisions that Willy has made in his question for success --- so that we can see rather than just think about.
In Arthur Miller's play "Death of a Salesman," Willy is the title character, though his name is not mentioned in the title. It is his death that results from the actions of the characters, and even in the subtitle, "Certain private Conversations," the conversations are Willy's and no one else's.