Not sure about the G.I bill however I do know that TA no longer covers it even though ISSA still advertises they do in their magazine ads. You can attempt to have it and numerous other certifications funded through NAVY COOL. Check it out for more info.
As far as ISSA goes they are reputable, I've had several of their certs for years but they only teach you the basics....even their so called master and advanced courses. Be prepared to learn as much as you can on your own. If you desire advanced knowledge then hit the library for a few weeks religiously and you can learn what takes most people 4-8 years.....and for free!
Just remember that whatever cert. you decide to go with it will only be good for opening a door or two. What will ultimately matter is your ability to assist a range of individuals in their individual goals. Dealing with people is a skill you can not learn from a book or a course. If you can do this already than the cert. is relatively unimportant.
Peace and good luck to you,
M2M
NASM is very reputable, but the contact ive had with the people from ISSA is phenomenal. as far as customer service and whatnot I've never encountered any company in regards to anything quite as good as ISSA.