In Death of a Salesman, what are some of the reasons that Biff and Bernard turned out the way that they did?

As a male, I
have a slightly different slant on this.  As a teacher, I have seen lots of "Biff"
over the years ... especially the "star" athletes who received special treatment in
almost all aspects of school life ... and please, this is not a general statement about all star
athletes, just something I have seen in too many cases.  It seems, because we don't know enough
to judge, that Bernard was a naturally better student; we know this becuase we have the evidence
of his success in law school.  Again, there is only "circumstantial" evidence that
Biff was not a good student; he regularly needed help ... but this could be caused by any number
of things.

The one thing that we know is different between the two is that
Bernard did not catch his father in an affair with a stranger; I think this is the turning point
in Bieff's life.  Biff had so much faith in and admiration for Willy that finding him with a
woman other than his mother wrecked his life, killed his chance of going to summer school,
getting the credit he needed, and attending college.  It's possible that, without this incident,
Biff might have had a life with at least some of the success that Bernard enjoyed.  Who knows. 
What we do know is that the trip to Boston contaminated the rest of Biff's life and Bernard had
no such reality to deal with.