Groundschool
Both schools are very good, I'd say it all comes down to your own personal situation. I'm with CATS at the moment and the flexibility and price of the course were the biggest deciding factors for me when deciding on which school to go with. I think the two week brush up before the exam week with Bristol is an excellent idea and have heard from friends there that it works very well, but unfortunately I would never have managed 3 weeks at a time (twice) off from work. Also the flexibility of your exams/module subjects/sittings has been incredibly helpful, with limited days off work it's been fine for me to just say 'Right Im going to do all the Monday exams this sitting' or whatever, literally whatever suits you best and the head of training and other instructors really do their utmost to work everything around for you.
I've never heard a bad word said about Bristol either though.
Totally depends on what's best for you, why not contact them both and get a feel for them?
All the best with your decision
C