Personally I think you are wasting your money to get both... I would get the ATP prep prior to your interview and study that -- understand the concepts behind the question, not just question answer familiarity. When you take the ATP question answer familiarity is the goal. There is a TON of overlap between the ATP prep and the interview prep and nothing in the interview prep that isn't in the ATP prep that will prevent you from getting a job at PSA. Good luck and happy studying.