I took The Princeton Review's course last year. Unfortunately I had a family emergency and coudlnt' write the test last August. I am taking their course again this summer. I was very impressed with my teachers (although I don't have the same teachers this year, so we'll see!). I heardt that PR actually goes through a very tough audition, recruitment and training process for each subject. It is actually possible that some people go through all that and end up not getting hired, which is pretty interesting. I am not too sure about Kaplan, but as far as I know they have some kind of "orientation/training session", which is not as intense as the Princeton Review. I don't know how they hire teachers.
Their books & online student centre support are fantastic. Hope this helps!
Gaby