Well being a bit of a training junkie I have done the following
CSS Level 1 - Rockingham
CSS Level 1 & 2 (2 day camp) - Laguna Seca
CSS Level 3 - Cadwell
ESS Blue - Snetterton
CSS Level 3 - Rockingham
Back to the question. First these are both very good schools and I learnt something new every time.
CSS is more structured, you start at level one, and have to take the levels in order, which some people do not like. I did not find this a problem.
Each level has five classroom sessions, with five track sessions immediately following where you practice a drill to re-enforce what you have been told. You work with the same on track instructor all day. So across the three levels there are about 15 specific skills that you will be working on improving. There are also off track drills
ESS also has classroom sessions focusing on specific skills, but a single class session might present more than one technique, and they do tie the track session to a specific drill. With ESS you do not have to start at the begining as they will let experince riders move straight onto the more advanced red and yellow courses.
Classroom sessions. CSS definatley has the edge here. It is not that the ESS classroom sessions are bad, it is that Andy Ibbott's classrooms at CSS are excellent, he is very good at getting the complex information across.
Track sessions. This was a much closer call for me. To a large extent it depends of which specifc instructor that you get. I prefer the structure of the CSS approach, but I got an incrediblely good on track coach at ESS. In a perfect world I wouldn have the ESS track instructor teaching at CSS
Overall I preferred CSS, but they are both good and it is quite close.
Thoughts
Experience / Fast rider
Starting at the begining at CSS. I have heard this question several times. In the end only you can decide, but although you will get the same class and drills, the on track instructor will be giving you different feedback and making you focus on different areas ie your weaknesses
Two day camps at CSS
These look very expensive, but trust me it is a completely different experience to the normal one day class. Twice the price, five times the experience
Rob