I am facing the same dilemma with Duke except with Maryland, although my interests are far more focused on social policy. I think you're making the right choice - in the end, if where you want to end up is negligible to the name on your degree, then why throw yourself into 45k worth of unnecessary debt. I'm a strong proponent of the notion that there really is no ceiling (irregardless of your alma mater) to achieving high level positions, albeit getting there is more often than not a lot harder to obtain, and is by no means as procurable in such a short period of time.
That being said, I'm looking at a 23k (including the 4k assistantship) of funding from Duke, and with external fellowships, I would be 34k in debt after two years including living expenses.
Maryland is offering me a full ride (excluding 2 credits) alongside a 20hr/week assistantship applicable to my interests with a stipend of 13k. With external fellowships, I could pay off my 16k of undergraduate loans (that are not deferrable) and graduate with no debt if I went to UMD. Then again, I also run the risk of graduating without a propitious initial job offer from Maryland juxtaposed to Duke's claim to strong networks in DC. I'm mainly concerned about acquiring a favorable summer internship. ::sigh::