I work at a YMCA & I’m thinking about getting a specialization in one of these.
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.