Aviation exam is an excellent tool, however there are some subjects which AE doesn't quite do it alone, namely flight planning. I got bitten badly in that exam with loads of Reduced contingency fuel and PDP calculation questions that I had never seen before ... AE only have a couple of theory questions on this, no actual calculations. Yet BGS had numerous questions on them which I only discovered after the exam. So yes use one mainly ... but do have a look to see what the other has that your main one hasn't covered. Together they almost certainly cover it all though.