The difference in the information and quality of those certifications is actually dependent on what you put into your studies. ACE, ACSM, NSCA, and NASM are the most recognized certifications when it comes to the applying for a job as a personal trainer in most markets. But you are the only one who can make a real difference in the quality of your certification. Being a personal trainer is so much more that passing a certification exam. The four I mentioned are the ones that I recommend when asked by people looking to start a program to become certified. Then I follow up with advising the person to seek out fitness professionals in their community and ask to observe them working. And if you plan to be a personal trainer, you should experience having some sessions with some of these fitness professionals as well. I have taken several of the certificaiton exams just to see what they are like. Most are very similar. My current certification and one that I plan to never allow to expire is with ACSM.
And one last thought. I do not take a personal trainer seriously if they do not spend a fair amount of time researching and studying the ever changing information in our industry. If you aren’t interested in continuing to grow and improve as a professional, how much do you really want to be in that profession?
Martin made some great points and that is how I would also approach my search if I’m looking for a certification. From the ones you’ve mentioned, ACE might be your better choice. For me NSCA/CSCS serves my interests better than any other one because of the type of clients I work with. Do your research on the certifications you are looking to go with and see which is the one that will serve you the best.