ATPL and BGS are like for like. Some of the explanations are as little as a letter 'L' for example - meaning Lift from the Lift formula. That is the explanation for 30% of the Principles of Flight Questions. That being said, these two sites do as they say they will, they give you practice questions and sometimes help you towards the right answer. They definitely don't teach you the subject, this is where Aviation Exam is different.
I haven't used it myself but a lot of my course-mates have and they say it is great. The questions are regularly updated and there are usually, and sometimes in depth explanations about the answer and the methodology used.
Although it costs more, it is probably your best bet if you are looking for maximum experience.