I’ve held NDEITA, AFAA, two levels of YMCA certifications, and two in-house corporate certifications for group-ex. When I got my certification as a personal trainer with ACE, I let the group-ex lapse.
I left my AFAA group-ex certification because of their renewal requirements to take another AFAA course, and they rarely come to our area.
I have the textbooks for ACE, AFAA and ACSM group-ex. ACE and ACSM are both quite thorough. I’ve held enough group-ex certifications that I don’t feel the need to pay for another group-ex test, but if I were to take another group-ex certification, it would likely be ACE so I could save $ in renewal.
While I agree with Martin that the ultimate quality of the instructor is not the certification but the effort and attention that one puts into learning, absorbing, and implementing exercise education, sometimes the certificate itself opens doors. In my area, ACE is more highly regarded than AFAA. ACSM is well known (as is NASM) for personal training, but not as many people even realize that ACSM has a group-ex certification again.