I think you will find Duke will blow away Northwestern in this poll, since this is SDN afterall and all you have on here is 95% pre-meds who only care about a list that gets released by US News each year to base their decisions off of.
You will become an MD and get a quality education either way. The curriculums, location, etc., are extremely different so it is difficult to compare these schools and even do a poll. Do you like 100% suburban or 100% urban living? Where would you prefer to be single and doing your residency? Durham or Chicago? The lifestyle you will have at Northwestern as a medical student will be COMPLETELY different than your lifestyle at Duke--from the location, to the apartment, to the lifestyle on the weekends and free time and competitives during school, to the hospital, to the people you meet and befriend and their lifestyle too, to the people you meet and possibly date in/around the area.
That being said, after living in the Triangle for 3 years as a UNC student, I turned down Duke to come here (Northwestern) simply because I was bored and depressed living in such a disconnected suburban area. Not once have I regretted my decision, because quite frankly I would much rather do my residency here at Northwestern than at Duke, and I have a much better chance going here. Not to mention my lifestyle is completely a 180 compared to how I was living in the Triangle (had to have a car, lots of sprawl, no street lights even on the major roads, very quiet, was honestly feeling pretty lonely, etc.). Chicago is one of the few world-class cities in the US, and if anything there is too much going on and too much to do--the opposite of how I felt in the Triangle. It is nice to walk a few blocks here or go out at night and be off of campus and see thousands of people living "normal," non-medical-student lives, for instance, rather than feeling like you are stuck in a bubble, and I can't tell you how this has saved me from going crazy countless times.
Go with the lifestyle you want to live, because that is your LIFE that you will live... every freakin' single day. Every freakin' day you will have to get yourself out of bed and face an entire day in either Durham or Chicago, with all the positives and negatives that come with each. The lifestyles are so very very different that it should be quite clear which one is the better decision for you.