Understand, Chicagoland is huge- I'm there too . Coverage can vary from block to block, so that will have to be trial and error on your part. In some places the indoor coverage won't be as good from one carrier or another based on the construction materials of a particular residence/workplace. That has nothing to do with the carrier unfortunately.
Even being on the opposite side of a tall condo bldg/skyscraper might mean you pick one carrier over another because you face the correct cell site. YMMV.
Best way is to find out what service your friend's/neighbors have, just to get a sense of coverage you need. ATT & Verizon will have very similar pricing when you get down to the brass tacks.
i currently have tmobile and the contract is up. i want the new iphone5, but i am not sure who is a better carrier, verizon or att in the chicagoland area. better coverage/perks??