When I was applying to medical school, I toyed with what I thought was the same dilemma. Now that I am in medical school, I will say, go with where your heart tells you. I thought that if I went with what I thought was "a lower tier school" just based on their admission requirements I would be almost guaranteed to be in the top of the class, and while I'm holding my own ground, there are plenty of people in my class who are doing much much better than me - some of these people are so smart, I would expect that they would have turned down acceptances or large scholarships to top-rated schools, but some of them (who are doing much better than me) didn't get offers from other schools (for whatever reason). That's not to say that you might not get to a "lower-tier" school and ace the courses, but don't expect that that will be the case. Go where you feel like you will be happy & get a good education. From what I have heard (I'm only a first year), your board scores & your 3rd/4th yr rotations have the most to do with your residency placement.
Hope that helps. Good luck.