This is an interesting question because, in Death of a Salesman, just when you feel that Willy and Biff are reflections of each other, we realize that they really are not. Biff is not a reflection of Willy: he is a consequence of Willy, perhaps, even a casualty. There are more differences than similarities between the two of them.
The first thing to consider is that, obviously, Willy's physical condition seems to be far below that of his son, Biff. Many times we find Willy re-emphasizing how his sons are built like Adonises. However, he clearly tells Linda how he, himself, is fat and how someone allegedly called him once a "walrus".
Willy: I'm fat. I'm very foolish to look at, Linda. [...] as i was going to see the buyer I heard something about-walrus. And I- I cracked him right across the face. [..] they do laugh at me. I know that.
Biff and Willy are also different in terms of determination versus ambition. Willy has the ambition, while Biff has the determination. While Willy hopes for Biff to make it big in college football (ambition), Biff gives his 100% to ensure that it happens (determination). He is even willing to go to Summer school to make up for his flunked Math grade. However, both Willy's ambition and Biff's determination go awry after the discovery of Willy's love affair with "The Woman".
Biff is the realist while Willy is the never-ending dreamer. Nothing can ever shake Willy off the ideas that he has built for himself, and for his family. Biff knows better. When he is kicked out of the Loman home and moves out West, he realizes what he really loves: the outdoors, animals, the seasons, and nature.
Willy also knows what he loves in life: building things. However, contrary to Biff, he buries his love for building and embarks in his sales career, hoping to make it the way Dave Singleman made it. As we know, Willy had all the wrong dreams, all along. Biff is even able to realize this much. Willy is never able to do so.
One thing which makes Willy and Biff similar is their willingness to do what is best for one another. They have a father/son bond that is unbreakable at first. Willy wants his son to excel and he even sacrifices his own life so that Biff can end up with the 20,000 dollars of Willy's life insurance.
Similarly, Biff (in his youth) basically sacrifices his self-identity and self-direction in order to please a father whom once the thought to be everything that Willy claimed to be. After the affair of "the Woman" is discovered, Biff realizes that Willy is a phony and that, as a result, his (Biff's) life has also been nothing but a fantastic lie created by Willy's ambitions.
Hence, fantasies are what make Biff and Willy alike. Also similar are their broken dreams, dead ambition, and lack of determination. When Biff commits acts of theft he does is as a reaction to not having anything concrete to hold on to each time he tries for something. When Willy plants seeds is for his lack of having anything laid "grounded" for his family. In the end, both men try to find a solution: Willy kills himself, and Biff simply walks away from the silly dream for good.