Hi Jo Ann,
I have read all three books (atkins, PP and PP lifeplan) and I think that protein power explains everything much, much better in terms of the science aspect of things. I also think that the PP books explain more clearly how to really implement this lifestyle on a daily basis. I just found the PP books to be more reader-friendly and accessible, so I would advise that you read them even if you do eventually decide to go Atkins.
Susan