Dear Graham,
The difference between both is that the CAPM Exam is solely based on theoretical foundation given in the PMBOK, whereas the PMP Exam focuses on situational questions. Situational by means of scenarios that PMP Exam takers should have experienced during their professional career. That is also the reason why a longer work experience is required as a eligibility requirement for the PMP Exam. Therefore, I would highly recommend you to acquire the Exam Simulator for CAPM. The PrepCast Tutorials are identical, thus the Exam Simulator would be sufficient for you.
In case of further questions, feel free to contact me.
Best regards,
Mustafa