I don't think you'll find many comments from people to have used more than one GS so getting an answer to a "which is best?" question is not really feasible.
Non of the GS are geared towards Airlines anymore than the JAA sylabus is. They are required to teach a range of subject that the JAA will examine you on. All the ground schools will cover the same things. The only possible difference in content that I could envisage is that some may home in on key areas that they think are examined heavily and lighten up on other areas, but I am not sure that is a viable view as nobody knows, except the JAA, what the question bank is.
If you do the ATPL full time then it maybe possible, with a fair wind, to do them in five months but you will be looking at about 6 hours study per day to acheive that (assuming you give yourself a rest at the weekends), which is one hell of a lot of work!! If you fail an exam at the end then add 2 months because of the required booking lead times.
I am using Bristol and at no point do I think they could have done it better. First class.
Good luck!
HWD