To summarize everything I hear on the subject and what is my impression after taking a look at the manuals, they're all very good and of course they all have their strengths and weakenesses...
Oxford - way too much in the books, but perfect as reference
Bristol - a bit too thin on some details, not all past exam questions are covered
PPSC - just a little too much, too bad the books are not updated (bankrupt)
All those little differences gradually disappear once you hit the tests, exercises, practice exams, feedback questions, etc.
It is my understanding that there's no precise guideline as to what should be covered in the course, explaining those differences. JAR-FCL says what subject should be covered, but not how deep you have to dig. It is left to the FTO to come up with something "acceptable" to the authority who, generally, takes a quick look at the books, keeps them a few weeks and then sends the approval.