When it comes to picking a certification, you want to go with the one that is consistently highly regarded where you live, and that will make you the most competitive you can be in that area, because you're probably going to want to establish a foothold somewhere, and keep growing in that area. The average client you'd be training isn't going to know the difference between the trainer certifications, and therefore won't think or care about it too much. However, it's good to have a certification that is highly regarded, in case they ask about it, or want to look into it more, or want to compare you with the next trainer because they're spending a lot of money and want the most for it.
You could think about pursuing a college degree in a fitness related field, such as exercise science. Not only will this expand your knowledge base, but it will give you a degree that most people can relate to. The average person isn't going to know what the better trainer certifications are, or the differences, but they will be able to relate to you having a degree, because they too are likely to have a college degree. This will help separate you from the guy who is straight out of high school.
If I'm going to pay money to hire a personal trainer, I want the best bang for my buck, or at least what appears to be the best bang for my buck. If you've got two guys who you've just met, they've both got good personalities and both know their stuff, then I'm still going to pick the one with the highest credentials. In the great scheme of things, credentials are primarily formalities; not the most important aspect of your personal training career, but still very important nonetheless. This doesn't mean you should go out and try to get multiple personal trainer certifications, like so many try to do, because studies have shown there is no positive correlation between more certifications and higher profit yields. Meaning, you're not really improving your name by putting more certs after it. Get one good certification, and a college degree if you can, and hit the ground running with your marketing approach. Most trainers neglect to learn good business and marketing skills, and therefore, they tend not to do as well as they'd hoped. Personal training is a business, therefore, business skills are needed. You can be the most knowledgeable person in the world when it comes to health and fitness, but if you don't market yourself, then you're never going to get very far because people aren't going to think about you if they don't even know you exist!
Check out this site. Lists the more popular certs and details them... Personal Training Accredidation