I've used both and like both. During the period from starting to study a subject in detail I use Airquiz to monitor my progress and to get an insight into how the questions on a given subject are presented. Once I think I'm ready to take an exam I sign up for that exam with PPL Cruiser too and use both to cram.
Airquiz doesn't usually go into detail on the answers (they do point you in the right direction if something has recently changed from the current text books) but they do email you the results and what you got wrong. I then go back over these incorrect answers until I understand why I screwed up. Also keeping the emails and reading back through them later on is very helpful.
PPL Cruiser give you immediate answers on each question with explanation and things to be wary of on some of the questions with tricky wording. I like that you can just do a few questions when you get a chance, which keeps the subject fresh as you lead up to the exam.
From my experience I think airquiz is better to help you learn the subject, however, PPL C is a very good cramming tool to get you through the exam.
Both, I think are good value. I paid for the whole package with Airquiz but I pay for each subject as and when with PPL C.
Hope that helps
