Are you planning to become a personal trainer or a physical therapist? Will you be working for a large chain gym or a private gym?
I just want to tell you that I've worked for 2 gyms (chains) and a YMCA and they really don't care where your credential comes from, just so you have one. The thing clients and managers are looking for is personality, dedication, reliability, confidence, etc. That's been my experience, anyway. And not one client has asked me yet where I got my educational background is.
I am going to take a 2-year course with UCLA Extension to further my knowledge and expertise, but honestly, one of the trainers I worked with had a lot of education and she failed as a trainer because her personality was just not there.
If this is your first certificate, I'd suggest something that is easier like AFAA or ACE, so you can get a feel for the tests.
ETA: I just did a search for CSCS to confirm my suspicion that it was a specialized degree:
"Certified Strength and Conditioning Specialists (CSCSs) are professionals who apply scientific knowledge to train athletes for the primary goal of improving athletic performance. They conduct sport-specific testing sessions, design and implement safe and effective strength training and conditioning programs and provide guidance regarding nutrition and injury prevention. Recognizing that their area of expertise is separate and distinct, CSCSs consult with and refer athletes to other professionals when appropriate."
I believe this is the same certificate my friend held and she said it was specified for athletes - it is rare to find an athlete looking for a trainer in a gym-setting.