As opposed to what klownzo said, I grew up in Milwaukee area but have lived in the twin cities for a little bit. While Milwaukee is not a bad city, I think the twin cities area was nicer. While it is not huge, it is bigger than Milwaukee and has more to offer in that sense. Both cities have the midwestern vibe. However, the twin cities has a more progressive feel. The population seemed a lot younger, more educated, more inclined to exercise and eat healthy, etc. I think this is due to the fact that a lot of college grads from the midwest move to the twin cities after graduation because of the job prospects (it is home to many large corporations such as Target and General Mills). I went to school in the midwest, and the general consensus I got was that the people I met that grew up in the twin cities mostly all loved it and wanted to move back. The people I met from Milwaukee were more hit or miss. Some wanted to stay but lots wanted to move somewhere else after graduation. Again, there is nothing wrong with Milwaukee and I have had good times there as well, but I feel the twin cities is a lot nicer and more fun. Additionally, I heard the job prospects for dentist are much better in Minnesota.
While I can't attest to either school, I have heard great things about both. I feel I have generally heard more people choosing Minnesota over Marquette than vice versa, but have also heard of people liking Marquette better. Minnesota is more expensive (though not by much), so it really comes down to whether you think the extra cost is worth it. To me, I would prefer to pay the difference to be in a more bustling city and to be right in the twin cities for the job prospects after graduation.