I agree that Biff is the only character who experiences growth. He is nearly as central as Willy, but - I would argue - not quite.

This is the story of the delusional nature of a man who has bought into a particular vision of success in America. Willy's failure is the central comment of the drama, in my opinion, and Biff's achievement of clarity is secondary to Willy's muddled failure to see himself as he truly is. 

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Argument for Biff as main character: 1. Biff is involved in the most dramatic scenes of the plays. 2. Biff undergoes change over the course of the play. He is the only character to do so. 3. Biff is the only character to speak meaningfully to all the major characters in the play (Happy, Linda, Willy). 

Biff can be considered the protagonist because it is he alone who grows as a character.

In the beginning of the play, he exists under the same delusions of grandeur that he father Willy has taught him.  He hopes for the same sort of different life other than the one he is living, the one he has created for himself.  Sharing a room with his brother Happy, Biff dreams:  "Sure, maybe we could buy a ranch.  Raise cattle, use our muscles.  Men built like we are should in the open." 

Biff sounds just like Willy too when he degrades his co-workers and situation, refusing to accept culpability for his own life:  "Sometimes I want to just rip my clothes off in the middle of the store and outbox that goddam merchandise manager.  I mean, I can outbox, outrun, and outlift anybody in that store, and I have to take orders from those common, petty sons of bitches till I can't stand it anymore."

But towards the end of the play, Biff has an epiphany.  He knows his father has been slowly committing suicide.  It is he who forces his mother and Willy to "see" it.  It is Biff who confesses his time in jail.  He comes to accept responsibility for his actions:

"I am not a leader of men, Willy, and neither are you.  You were never anything but a hard-working drummer who landed in the ash can like all the rest of them...I'm not bringing home prizes any more, and you're going to stop waiting for me to bring them home!"