I used Bristol for stage 1 and they were spot on. However from recent feedback on performance, mass & balance, instruments and Flight planning a lot of the questions that came up in the exams were not in the Bristol question bank, however I heard that aviation exam was spot on for each subject in terms of "seeing the question before" so I would recommend using both just so you have all basis covered.
From what I hear is that aviation exam has excellent explanations so your not just memorising answers but learning at the same time.