(I’m self-studying) having trouble understanding cliffs…so I ordered Princeton just now. Not sure it was the right decision because I’m wasting a lot of money on AP Exams already as it is 
the 2015 princeton
I’m using the 2015 Princeton Review book to study as well and I think it’s really helpful! Definitely not a waste of money. It’s very easy to comprehend the material and the practice tests are great. A lot of people prefer Cliffs, but I like Princeton better because of the way the chapters are set up. Good luck. 
Has anyone tried the 2015 Barrons yet?
Using 2015 Barrons right now, seems very nice, pretty easy to understand…really seems to correlate with Campbell’s textbook but simplified without extraneous crap. Also, the content is really organized, chunked into sections, and with really great diagrams. I know that some of the complaints for the 4th edition Barron’s was that it didn’t reflect the revised AP Exam, but I think/hope the 5th edition (2015) is now more based on the new test. Also, when briefly looking through the practice test questions, the questions seemed to reflect that paragraph of a question format collegeboard seems to be following. You should seriously consider it: it’s definitely reader-friendly.
Update: I read both Princeton and Cliffs and watched Bozeman Science videos (all of this mainly in the week of spring break) and a few times after and got a 4.
I really recommend practicing the past Free Responses on College Board…I didn’t get to do that
Oh, and I took notes when watching Bozeman Science Bio vids