I bought both cliffs ap and shaum's bio but I have only read through Cliffs. I skimmed though schaum's and it looks very good, lots of figures. Cliffs did a nice job going through each section and then has questions after each chapter.
I got a 24 on the Bio and I studied Campbell's bio book and an older Kaplan book. Campbell's was very helpful. Go to the ADA website and look at the breakdown of questions they ask (e.g.- 13 from microbiology, 5 from body systems, 5 from cell metabolism, etc...) and study these sections in the Campbell book and you are guaranteed to do well.
Be sure to try and understand the bio, don't just read it, go back and review everything and answer the questions. Only when you can really explain it to yourself will you know that you mastered the material. I think the other books give a great review, and would recommend them , but read Campbell's and you will have a great base from which to build.
use campbell's b/c it is extremely detailed and very thorough. it is also used for colleges and has many updates and changes that seem to correlate to the science sections on the dat. that's just my opinion.