I would take Michigan without a second of hesitation for the following reasons:
1. Michigan's reputation is consistently and undeniably stronger than Duke's. Let's face it, U of M law is among the top top top law schools in the nation, and part of a truly world class academic institution. Of course, Duke is still a great school, but Michigan is simply in another class. Lawyers, judges, and professors agree.
2. Facilities- Mich's law campus is jaw-dropping. Living in the Lawyer's Club seems surreal...could you imagine eating in that cafeteria every day? Duke, on the other hand, has among the worst facilities of the T-14.
3. Student body- I'm not very familiar with Michigan's student body, but I can tell you from first-hand experience that Duke students tend to be pretty homogenous and relatively conservative. Seems to me that Mich is quite a bit more liberal/open-minded, which would be HUGE for me. On a similar note, I constantly hear about how Duke's student body is pretty apatheic and are strongly steered into the biglaw thing. Mich on the other hand is always played up as being more intellectually vibrant (though most of their students also pursue biglaw).