Aviationexam tend to be biased towards the european-wide exams, whilst Bristol tend to concentrate on the UK database, which has been modified due to some hard work by the CAA and the CGIs over the past year - all the questions that have been put on hold over the past years have now been inspected and modified and made available again, or flagged as useless as necessary. As such, the UK version of CQB 13 is the most logical of the lot.
Otherwise, the only difference is in the method of usage.
Phil