I believe that AviationExam is more useful for European-wide ATPL examinations, it has a much larger QB to account for this; whereas the ATPLOnline QB is more representative of the UK syllabus examinations as its based on feedback from it, although there is a small number of euro questions in it, they are tagged. I think they recommend focus on the V2 questions, and do v3 if you have time. Both ATPLonline and AviationExam are useful revision aids as they have questions based on identical syllabi.
Good Luck
