Unless you absolutely want to work in Wisconsin or Marquette is substantially cheaper than Iowa, go to Iowa. Don't worry about libraries, sports, rankings, faculty, etc. The only things you should worry about are your job prospects coming out of law school and how much are you paying for those job prospects. Are you going to get a job, and if so, is it going to be in a region you want to work in? How much are you going to have to pay? This should be your focus. Law school is three years of your life and ultimately you won't really care how much fun you had tailgating and how cool your professors were or how many books and law review journals are in your library when you graduate with $150k in debt and no job. You should be doing everything in your power to avoid that situation, and it starts with where you choose to go to school.
Iowa places better than Marquette in general, and it places much better in states/cities like Iowa (go figure), Nebraska, Minnesota, and to a lesser extent Chicago, St. Louis, Kansas City, etc. I imagine Marquette places better than Iowa only in Wisconsin.
Don't want to derail thread anymore or anything, but I feel I should also suggest considering whether law school is a good idea at all. A fair number of students of the C/O 2013 seem to have done well as far as 2L summer employment goes, but I know a number of students who did extremely well and who are exceptionally personable/bright/interesting and somehow have struggled. It's very tough out there, so just understand there is a chance you could do everything "right" and end up with not much to show for it.
So yeah, best of luck, and if you do decide to "take the plunge" then between Iowa/Marquette definitely go with Iowa

. I know I said job prospects are the only thing that counts, but if you find yourself in a position where it's close then you should know that Iowa attracts an awesome student body and it really does have a laid back/fun atmosphere to it. Law school will suck at times no matter what, and it's during those times you'll be thankful you go to Iowa where everyone is so supportive and easy going.