Chicago, which my wife and I have seen in the recent days, has less to offer than New York City. It seems the city has a strict anti-graffiti policy. But there are exceptions of course.
The neighborhoods Wicker Park (wikipedia) and Bucktown in Chicago´s northwest remind me of Williamsburg, a neighborhood in the New York City borough of Brooklyn. There are a lot of old and abandoned factory buildings and storage depots which are now used by artists and are home to restaurants, bars and pubs. This environment seems to attract a lot of artists and other bohemians.
There we met Ken Aiken, one of the local artists who participated in the original Chicago Cow Parade. He was working on his cow sculpture which you can see here.
Street art rules.