If you really want to be close, go with the Westin.

The Peninsula is only 3/4 mile from 100 W. Wacker; although obviously it can be nasty (hot & humid) in the summer, it is possible that the weather could make that walk very pleasant. If not, it is a very short hop in the car. If you're going to be coming off of a lot of travel and want to guarantee a hassle-free stay, that's the way I'd go.