I just came across this excellent file which provides a side-by-side comparison of six certification options: Fitness Resource Associates (FRA), National Exercise Trainers Association (NETA), American Council on
Exercise (ACE), Aerobics and Fitness Association of America (AFAA), and American College of Sports Medicine (ACSM).  This document is a great resource if you are struggling to determine where to go to get certified to teach group fitness.  It is just a reference, and does not include every certifying body available.   But it is a great way to see you major choices in an easy to read format.  Bonus if you live in the Boston area — it includes reference to when these companies are offering workshops in your area! 

http://www.ultimatebootcamp.com/static/contact/UBC_certification_review.pdf