I go thru the BGS a couple of times till I start getting 90% or so and then I go thru Aviation Exam once, maybe two times, try to focus a bit more on the flagged questions, but I would say that if I would do only BGS, my results would be probably the same or maybe 2-3% worse tops... As somebody said, the last 200 questions are quite helpful, you can find them on the atpforum. Good luck.