I agree with Susan that Karin and LaRue have hit on perhaps the most important point for you to consider when deciding with certification you should pursue: what population do you anticipate working with the most?
I am familiar with ACE, NASM, NSCA and ACSM. All are excellent with a specific focus on a particular population of potential clients.
It all comes down to the type of training you are looking to do. There are many great certifications out there and each one offers a little different option than other ones. I like to work mostly with athletes and people who have depend on being in the best possible physical condition so they can perform their job (law enforcement, military personnel, firefighters, etc.), so I chose the CSCS certification. But each trainer has different goals and target clientele which he/she wishes to work with and so they have to decide which certification(s) will be the best choice for them. A certification by itself is not enough to make someone a great trainer. They need a combination of education, experience and a great attitude as well.
there are many certifications to choose from, but i researched a lot before i chose my mine, and i went with NPTI, it is not well known “yet” but what i like about it was that there is a lot of hands on experience. you have your theory part, your nutrition and your gym sessions where you will be training other participants in the course as future clients. the gym membership will be included in your tuition, and you will be joining a local gym. well that was here in NC but some NPTI schools have their own facility. you will learn how to apply what you learn in the class. every session will be different than the next. the instructor will train you how to deal with different cases in cleints, so you wount feel any restrictions when training them.
hope this helps
best of luck
Another point to consider is whether you will be working for a health club or going solo. When I first started I worked for a local health club for awhile. If you do decide to work for someone else, such as a health club or gym, then I definately agree with Karin’s answer above. You’ll find that ACE, ACSM, NASM, and NSCA certifications are the most widely accepted of the many certifications out there.