I took the CSCS, mainly to get the insurance and I don't regret it. To be fair to the NSCA, they do cover some important physics terms - like moment arms and class 1-3 levers, but unfortunately they fail to apply this information to barbell training in a way that would benefit a prospective trainer. The Essentials of Strength and Conditioning text covers a lot of broad material - (like micro details of the chemistry of muscle building) - that may be interesting, but will not carry over to helping trainees. I looked at it like taking an intermediate level college course.
I believe that the mechanics information from the SS seminar will be leaps and bounds more useful in terms of making me a better coach. Of course, SS is much more narrowly tailored than the NSCA text. Goal of SS seminar: Lift correctly, teach correctly. Goal of CSCS: Prove a broad base of fitness knowledge.