Hmm I can help provide some insight on the GAC front, as I currently work there. I've met a lot of people at GAC and they have either gone to NPSIA or GSPIA - NPSIA being the highest. In fact, I went to an event a few weeks ago and it was literally all NPSIA grads/coop students. I haven't seen or heard of anyone holding an MPPA there.
See, my worry with doing an MPPA is that I feel it just screams government, mind you, this is not necessarily a bad thing. However, I am hesitant on picking a degree with a somewhat narrorw-ish focus. For me, international affairs opens up more doors, and its a bit more specialized. You can work for international consulting firms, the federal government ( not limited to GAC btw), international organizations, international think tanks, private sector jobs, etc.
I haven't heard much about the MPPA coop program. I had questions about NPSIA's but after working at GAC and meeting so many coop students there... it seems like a lot of people are getting solid coops.
HERE IS A TRICK: Go on LinkedIn and creep the program you're interested in. A bunch of people who are currently studying/have studied at that program will show up and go creep on their work experiences.