I believe the PEP exams were an ancient name for what's now called the ECE exam (Excelsior College Exam). I may be mistaken though, but PEP sounds familiar...
If you go to Excelsior College's website, they offer information on their various exams.
I would assume that the AP exam is going to be harder than the CLEP, mainly because instead of just getting a "pass", you want to get as high of a score as possible. If you can find a content outline for AP Biology, you can compare it to CLEP Biology's outline:
http://www.collegeboard.com/student/...ep/ex_bio.html