Looks can be deceiving.
In this picture, it appears the office towers of downtown Chicago are just a short distance beyond the edge of O'Hare Airport.
In truth, the driving distance is about 18 miles, and the commuting time is often an hour or more each way.
So if you'll be spending most of your time in the Loop area, it might not make much sense to stay in a hotel near the airport. You'll cut your effective time in the city by about two hours each day.
Search engines will produce some excellent room rates for the area, but ask yourself how much that savings will cost you in wasted time.