As a native Chicagoan (home to the world's first skyscraper BTW), the first question to ask is WHICH tour are you taking? There are double decker, boat and walking ones that I am aware of in the city. And don't forget the observation decks at the Hancock Building and (Sears) Willis Tower too. One thing to remember on any tour, is that you will be at the mercy of the tour guide's schedule and mode of transportation. You may not have much of an opportunity to change lenses or compose shots. When I find myself in that situation, I usually try to limit myself to my Sigma 10-20 and Nikkor 18-200. If I don't have the 18-200 with me, the 17-55 will be my primary lens. My guess is that your 18-55 will be yours.