Well, first I would like to add that I have not had a campus tour at either place. However, I have had relatives who have ended up at the Mayo clinic and if I were you I would go to Mayo. The reasons are this: Mayo is THE BEST hospital system in the world. I know someone (probably from the East Coast) is going to argue differently, but facts are facts.
When the King of Jordan needed chemothearpy where did he go?
For an MD/PhD I would be interested in their lab/research buildings which I have also not seen. Also, from what I remember Mt. Sinai has about twice as much national research money. But, that is something you would have to judge for yourself. More does not mean better and/or more focused. Oh also, you may become concerned with living in Rochester, MN (it is not NY), but if you are looking for a career spingboard...Mayo.