In many respects, I think that this becomes one of the central issues of the play.  The desire to sell the piano or keep it represents the notion of discarding the past or being tied to it.  Boy Willie sees the piano as a commodity, refusing to acknowledge its intangible connection to the past.  Berniece features a complicated relationship to the piano.  She steadfastly refuses selling it, but she no longer plays it and really has not acknowledged the role the piano plays in her present and future based off of her past.  In this dynamic, Wilson suggests that the past is something that requires some level of complex understanding.  The past, in the form of the piano, becomes something that has to be understood, something that Doaker Charles illuminates with his story of the piano's creation and what is represents.  In the end, I don't think that Wilson is seeking to force an arbitrary choice between Boy Willie and Berniece.  Instead, he is suggesting that the past has to be understood and acknowledged, and carefully negotiated into the present and the future.  The past cannot be effectively airbrushed.  Berniece demonstrates this by playing the piano in the end, and Boy Willie shows this relenting on his insistence to sell it.  In this light, the piano's representation of the past is seen in a way where both characters come to change, as opposed to a narrow and binary choice between the two characters.