ACSM, ACE, NASM, and NSCA are all accredited by the National Commission for Certifying Agencies (NCCA). NCCA accreditation is generally held as the standard for the field—certifying bodies must undergo comprehensive evaluation and regular renewals to keep this accreditation.
You kind of answered your own question as far as what is different about AFPA and the main/better known Personal Trainer certifications. To me, what you do before and after you get certified to be more important. Which cert you possess doesn’t make you a good fitness instructor. How much you put into being a good fitness instructor is important. I chose to be a fitness instructor and coach because I love doing these things. I want to improve lives and I thirst for knowledge in this field. If you don’t want to learn and keep learning, then you just want a job/income. Making a living is important. But careers like physicians, dentists, fitness instructors should be undertaken by those with passion for what they do. That is part of why all these careers require continuing education. Be passionate and never stop learning. Words to live by.