I have the NASM CES and a few ACE specialties, even though not ACE Functional Training.
I saw that you primary personal trainer certification is with NASM. For that reason alone, I would recommend the NASM CES certification. It uses as a base the OPT model and thus provides a good continuum to your existing certification.
While I like the information I got from my ACE certifications, they do not operate in the same framework, at least not the ones I have.
Hope this helps.
My personal training certification is with ACE and I have several specialty certifications with ACE as well. I have CES with NASM and with Justin Price’s The Biomechanics Method. I recommend that you go with NASM or TBMM rather than with ACE if you want to learn how to design a corrective exercise program for your clients. In my experience, ACE courses provide information that is just too general to prepare you to perform a proper movement assessment of your client. The NASM course gives you very specific training to help you understand which over active muscles cause your client’s knees to move in or out, cause an excessive forward lean, or an overly arched lower back. Then you learn which muscles must be strengthened to help the client to correct the muscles imbalances you find during the movement assessment. Both the NASM and TBMM courses have been a great boost to my career in fitness.
I have looked into the NASM Corrective Exercise program myself and would go with that over the ACE Functional Training Cert (I do not have either, but hold ACE Personal Training and Health Coaching). I do like ACE certs., but NASM seems much more involved and specific with regard to the corrective exercise training. You will be able to use the principles with just about every client.
Also, as a suggestion, if you are considering NASM you should also consider Justin Price’s Biomechanics Method. It’s worth reviewing both of these programs.
Good luck with your decision.