I have recently completed the Theory exams for the JAR ATPL and was lucky enough to have access to both the Oxford and Bristol JAR feedback questions, although i completed the ground schooling at Oxford. (Don't ask how!)
I must say that the questions were very similar although Oxford seemed to have a few more whilst Bristol had helpful explanatory notes to help in deciphering how they arrived at the given answer. (At Oxford it was a case of ask the subject specialist, also helpful).
Whichever you choose it will be a lot of hard graft but it is most satisfying when you are finally done & dusted, have a grasp of so many new topics & can really begin to learn more about flying.
Safe Landings,
Nev1.