See, I've heard others say the exact opposite--to avoid the large chain firms as it's easier to find personal good teachers on an individual basis and it's just luck of the draw with the big ones who just fill bodies and can rely on people coming to them because of their name.
Really I think your best bet is to talk to people in the area and get personal recommendations for drivers. Going with a company name is no guarantee you'll get a good teacher, neither is going with a small company. I asked around work and with friends and got the name of a guy who was a bit odd, but an excellent teacher, and I passed on the first go. It's about finding a good teacher, not just a brand name.