10-15 without any doubt..
Leaving aside tangibles such as the loan amount and post MBA salary, I think there are other intangibles factors that will turn out to be important during your MBA and even post MBA.
1. Peer group - The peer group at a top school will be definitely better than at a low ranked school. Sure, douche bags exist everywhere, but it is a safe assumption that the ones in a higher ranked school will be more ambitious and talented. I believe that in a degree such as an MBA, your peer group plays a huge role in shaping your experience. These are also the people who will be a part of your professional network down the line...Purely for this factor alone, I would choose a higher ranked school (assuming the difference in ranks is significant as seems to be the case here)
2. Post MBA career support - The post MBA career support offered by schools drastically dwindles as you go lower in the rankings. Although one may never require it in the post-MBA career, I would say that it is a pretty great nice-to-have.
3. Faculty - The faculty at a 10-15 school will be definitely better. I often find the argument that education you receive anywhere is the same as quite amusing. Faculty in top schools are real world practitioners who have been there, done that. They will teach you the same concepts that are taught in a lower ranked school, but their personal insight is what will be unique and eye-opening. That is why people choose top schools to receive quality business education.
I could go on & on regarding such factors that will directly or indirectly influence your MBA & post MBA life, but I guess you get the gist of my point. 10-15 it is, without a doubt!!