Both are great.
I have ACE personal trainer, but am studying for NASM. I love the OPT model.
Project out a few years and think about where you want to be. What certification does the future you need in five years? The reason I ask this is because I got ACE, and it’s been a great cert. I thought it was all I would ever need. By and large it has been, but there have been two situations where NASM was preferred; in both I got the job because of experience and other specialty certifications, but I did feel at a disadvantage.