I personally think there are more similarities than differences, so you might want to think about the ways in which they are actually rather similar as well as being different too. However, there are some distinctive differences that are well worth drawing out.
The first is that obviously Willy is completely delusional. The way in which he is presented at the beginning of Act One in this play indicates that he has rather a fluid relationship with both time and facts. Consider the way that he contradicts himself so blatantly, in the space of a few seconds, saying, for example, that Biff is "a lazy bum!" on the one hand then saying "There's one thing about Biff--he's not lazy." Also, think of the way in which he slips back into the past to retreat into a happier existence.
Biff is a character who finds it difficult not to steal and also does not seem to be able to find success in his life. He has just worked in a series of farm jobs and has never managed to get the kind of city job that would give him prestige and success that his father would like for him.
Finally, Happy, although he does have a job in the city, is trapped in a low position and is left waiting for his boss to die. He deliberately sleeps with the wives and fiancees of his superiors and is very much lost in his life. Note what he says in this quote:
But then, it's what I always wanted. My own apartment, a car, and plenty of women. And still, goddammit, I'm lonely.
Happy therefore, in spite of his apparent success, actually leads a very hollow life, as he identifies in this quote.