Hi
I am with CATS currently and am really pleased with support I have received.
So far am on a 95% pass rate average, I know people who did BGS and they have given great feedback also so to be honest its a choice of if you want the flexibility of 3 sittings and 3 1 week GS' or 2 sittings and 2 2 week GS'. The flexibility was the deciding factor for me as it would have been difficult to get long periods off.
Which ever one you decide will be OK you just have to put in the hours.
Best of luck with exams :-)