Having worked the previous two summers at Michael Best in MKE and having a feel for the Milwaukee market, I can tell you that the personality of the two firms differ greatly. Therefore, in my opinion, the firm that you can identify more with: regarding the individuals that you met and the atmosphere of the firm, is probably the place you should end up at(do they have open door policies, are people smiling, do the assistants look upset, etc.).
Both firms have a good reputation in the city, will provide good work for big clients, etc.
Regarding Michael Best, the reason it really stood out to me, was that it is not your traditional stuffy law firm. Attorneys are animated and diverse, there is an excellent level of comradery that transcends practice group, associate/partner, etc. Additionally, summer associates and young associates get REAL WORK. While intimidating at first, I think the success of MBF's summer program is the number and sophistication of projects and responsibility you are given. This gives you an excellent picture in determining whether you like the work, the people, etc.
Ultimately its a gut feeling...for me that was MBF.