I too narrowed my choice of groundschool down to BGS and OATS. I chose BGS on personal recommendation and have no regrets. Have since met many people who have been there and never heard a bad word about the school. In contrast have heard good and bad about OATS.
Bristol is a small(ish) school with a friendly atmosphere and great instructors and you'll enjoy your time there if you go. However the choice of school won't get you results, very hard work will.