What are Biff and Willy really arguing about?
Every discussion between Biff and Willy are really about Biff's dissillusionment after he discovered Willie with the women in Boston. For Biff's part, he is trying to deal with the fact that his dad is just a person, that he made mistakes --- that he was not (could not be???) the idea that Biff had of him. If you're familiar with Hawthorne, it's similar to Goodman Brown and his inability to accept people as less than perfect.
For his part, Willie always finds Biff "spiteful" --- Willy seems to think that Biff should be willing to forget, to take a more forgiving attitude toward his failing. He thinks that his explanation, that he was so lonely, should make sense to Biff, but it doesn't.
All their interactions seem tainted by these two opposing forces: Biff's inability to accept his Dad's failing, and his Dad's inability to understand how seriously this damaged his son. This isn't always the argument on the surface, which is usually about work/success all the things that Willie thinks are important, but it's the underlying agenda in all their conversations.