I am currently using CATS and have been since April this year.
Although I cannot comment on the quality of their ground school instructors (as I haven't been to a ground school week yet or had any questions - but I've heard it's very good) the quality of the content is great and easy to learn.
They let you get on with at your own pace with basic direction of what to study what week, how long it should take you to do each subject etc. Beyond all the material, they have things like key facts which you can go through for each subject and of course, practice questions and progress tests.
The main reason I went for CATS and not a provider like BGS was because of spreading the exams out over 3 sittings rather than 2. Due to work I am unable to take 3 weeks of work off at a time but CATS allows you to do them more spread out and personally that works better for me (plus Luton is down the road for me)
I have however, bought the BGS question bank as it's one of the best and acquired the Oxford CBT for harder subjects such as Met, GNav to help.
I know a few people who have used BGS and haven't got a bad word about it, so I guess really it's down to personal preference. Think about which structure works best for you, if the location has a input as well. I paid I think about £1300 for CATS for all modules including ground school weeks before exams, plus £140 for question bank and I think BGS is around £2100?
Let me know if you have any more questions about CATS from a student view, happy to answer any.