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.