What are the Pros/Cons of using the Montgomery GI Bill vs. the Post 9/11 GI Bill while still on Active Duty??

Why are you or would you use your GI bill on active duty?

I recall TA as having specific loopholes to jump through - you are not guaranteed assistance. I think you have to be accepted to a school, and your performance evaluations will be checked (including negative pg 13 info). There are limits to assistance, if memory serves: be sure to check your budget if you need to supplement course fees, special equipment, textbooks (if you have never been to college these can be prohibitively expensive, depending on your situation). The GI Bill just requires filling out paperwork and passing the eligibility cert to the training provider's vet rep or authorized handler. Be aware you will NOT get the living stipend if using gibill funds on AD.

It is very important to note that if you are likely to be deployed within 6 months of starting college coursework, or working hours consistent with a high operational tempo, I would find out exactly what the policy is for military personnel and incomplete course grades. If approved for assistance, you don't want to use benefits only to be shipped out prior to completing the requirements or working so many hours that you can't reliably learn the material/pass exams/complete projects:
you risk being denied future TA assistance (assuming they dont require signed notification from your CoC that you can devote the necessary liberty to completing the work), and in some cases with GIBill funds you could receive a bill from the VA ordering you to repay all tuition & fees for incomplete courses. The VA regional office that sanctions your attendance and coordinates tuition & fee remuneration to the school will have access to grade data and whether you dropped, or didn't complete courses. Federal debt can affect your clearance status, credit score, federal employment eligibility...

Can't speak to the MGIB, because I used the loan repayment program, but I did qualify for Post 911, its awesome. My grad school is being paid for and I get a book stipend and the monthly BAH allowance (prorated depending on # of credits). The process to certify my eligibility was rather lengthy, but that was due to certain unusual circumstances on my end that took VA some time to work through, but the process wasn't difficult.