Both are great cities and it depends on your interests. We enjoy spending a couple days in the miracle mile area for some nice restaurants and a great selection of hotels. It seems to me like they make better use of their lake front area in Chicago. I have spent more time in Chicago so I am a bit biased.
Oak Park is also close by and has the largest concentration of Frank Lloyd Wright buildings anywhere. He started his career in Oak Park and the Robie House is on the campus of the University of Chicago. The Johnson Wax building and Wingspread, two of his great works are nearby in Racine WI.