I second what collette said. It's up to your personal preference as I am sure that both are adequate.
I chose CATS because of the lower fee and the shorter brush-up course. I was on the fence regarding Bristol, but did not want to take 2 weeks off for the course every time.
I also did what collette did, and went to the exams 3 times. The first time I followed CATS stage 3 with 3 exams. The next 2 times I took all the exams on Monday/Tuesday/Wednesday & Thursday/Friday respectively.
If I were you I would choose the one that you are most comfortable with, as both deliver the necessary quality.