What is the difference between the Post 9/11 and Montgomery G.I. Bill? What makes you eligible for each?

If you enlisted after the Post 9/11 Bill was enacted, you should automatically be enrolled in this program. The Montgomery GI Bill had to paid into starting in Basic Training. The big difference between the two is that the Post 9/11 pays the school based on "months" and pays the member BAH based on single status at I believe the E5 rate for the area. Check with your education office, but you should already be enrolled in this program (and as a bonus, if you are looking at going into a STEM field, there is now an additional scholarship offering an additional 9 months of training up to $30,000)