I am a recent graduate of the Georgetown Public Policy Institute, having received my MPP last year. I was also admitted to the Harris School at the University of Chicago and GWU.
First of all, DO NOT make the mistake of relying on the U.S. News rankings to make your decision. I conducted significant research into MPP programs on my own behalf, and I assure you that the U.S. News rankings for schools of public affairs are absolute garbage. For example, I'm originally from the Midwest, and so began my search for a grad school in that region. The U.S. News rankings place both Indiana and the Ford School at the University of Michigan in the top 5 for public affairs - both Georgetown and Harris are in the teens (13th and 15th, approximately). Insane! Both Georgetown and Harris-UC are dramatically better programs than Indiana and Michigan. While both GU and Harris are more selective than UM and Indiana, that's not really the basis for my argument. It's more that Georgetown and Harris really focus on offering intimate learning environments with absolute top-notch faculty and lecturers - perhaps this is where reputation and location really do make a difference. Indiana is at best average.....they don't really offer many intangibles. The Ford School is weak......no thesis requirement, average faculty, and they just don't seem too invested in their students. (I had a difficult time just trying to arrange to speak to some of their administrators and faculty)
Regarding the question of selectivity raised by several posters - Georgetown has an acceptance rate of about 20% for its full-time MPP program. I believe that only the Kennedy School, the Wilson School, and possibly Berkeley have lower acceptance rates(can't speak to Columbia - which is a little different). Harris-UC accepts about a third of applicants, and GW takes about half. Hope this is helpful for some of you out there......best of luck (and of course, I highly recommend Georgetown )