In the e-mail it didn't specifically state a deadline, but I think they give about two weeks for you to accept.
Chicago has by FAR better research than Sinai, but I did love my visit to Sinai much better than U of C and the area is a LOT better (in my opinion)...
...but Chicago is a nice city as well, so if research is a priority and you are ready to venture off to Chicago, I would chose U of C. In terms of residency options, both will open up doors to great places (and don't mind the naysayers about the Sinai's hospital system troubles because that will probably mainly affect the hospital residencies (but only in the short term)).
I hope I didn't make your decision any more painful 🙂