It sounds like you've done your preliminary research on the various agencies out there. The number of available options can definitely feel overwhelming, especially when each one seems to have one benefit or feature that another might not.
If I were in your shoes, I'd keep the following considerations in mind to help guide me in my ultimate decision:
How do I want to define my scope of practice?
-Do I want to be certified to provide general advice to clients about nutrition?
-Do I want to become a registered/licensed dietitian to be able to provide more specific nutritional programs for people?
Where do I want to work and what are their qualifications?
-Do I want to work for someone else and their company's rules?
-Do I want to open my own business and make the rules based on what's legal/ethical?
What's the fitness/nutrition industry like in the city/state I plan to work?
-Should I jump on board with what seems to be popular?
-Should I "break from the herd" and take advantage of a unique market opportunity?
If you'd like me to clarify any of the above please let me know and I'd be happy to.
No matter where you find yourself currently, I'd recommend you seek out advice from local businesses who provide fitness/nutritional services. If you haven't done this already, ask them their thoughts on your situation and what their recommendations are.