Not many people will be able to give you a balanced answer on this as people have only every done one groundschool!
I have gone with CATS, great price, great notes, and they are always developing new ideas to teach. Stuart is a top teacher and I throughly enjoyed his lectures.
Couple of questions you need to ask yourself -
Distance, are ethier school commuatable, as accomodation for brush ups adds to the price, its also nice to be able to pop to school of you have any issues.
Brush up weeks, will work allow you 3 weeks in a row of for bristol? ( 2 weeks brush up then a week of exams? ) CATS is way more flexible in allowing you 3 modules, in any order you wish. They are sensibly grouped together in a logical way, which is a bit easier to get to grips with then sitting 7 exams a time at bristol.
Price - CATS are great value.
I chose CATS, got an old set of oxford manuals for £50, and use the bristol qbank. WOrked a treat for me so far!