I went through NASM. From what I saw, it required the most understanding of the body and nutrition, including requirements for certifications in CPR and defibrillator training. The others didn't seem as "science intense" or completely anal about this stuff, which is why I chose NASM.
I may argue that ISSA does require CPR/AED, and first aid.
And most gyms won't let you make nutritional observations on clients unless you have a degree in nutrition, not just part of your certificate.
NASM isn't any more science intense than the NSCA either. I went through the NSCA already, and my current college program uses NASM texts and certs. The information is very similar. You could probably damned near pass one test by reading the text for the other. I think most of the information is the same, at least throughout the ISSA, NSCA, NASM, ACE and ACSM. I think they all require CPR/AED certs.