Wonderful question.
What you first should do is look at the student reviews for the hospitals at the school and email the students that have rotated there.
One thing I NEED TO EMPHASIZE...If you are planning on doing surgery or EM or IM or vitually anything else, you should rotate at a facility with RESIDENTS and INTERNS! This is a must! You need to learn how residents work, and what they need to know to succeed...and the best person to ask is a RESIDENT. Dude, they were JUST in your shoes...they know what you need to do! (example, in the "House of God" or the cheap rip off tv show "Scrubs", who is the best form of info...the resident.)
Residents will give you hints, they will give you pointers and best of all, they will make mistakes too. That allows you to relax, and judge your level of developement.
Also, one must give up the idea that "rural hospitals allow students to do more". Good teaching hospitals are just that, teaching hospitals!
Choose a med-large site (200-500 beds) and get a feel of it all. Otherwise you MAY enter your 4th year with very little working knowledge of how "the great residency machine" operates, and how/what to expect from residents.
(Usually I add sarcasm to my posts...but I will hold off for you)
Michigan sites are always solid, and so are many east coast sites. The Fla sites are "iffy" from what I hear.
good luck.