They all suck.
The NSCA supports research more than the others, as they have their own journal. This is reflected in their textbooks. NASM sounds good with their corrective exercise focus, but their reading material is soft on content.
If you're interested in corrective exercise, you may want to get NASM, as some specialty courses that are actually good will give CE credit for NASM. I am certified through the NSCA, and besides courses taught by Stuart McGill, I haven't found any of their CE courses appealing.
They basically all suck in different ways.
There are other threads somewhere that also talk about the different certifications and talk about becoming a Personal Trainer.