Definitely - without question - the Renaissance Downtown for a leisure stay. The Renaissance Schaumburg is also a fine property if you have business in the NW 'burbs, but for a leisure stay it wouldn't be all that convenient: you'd have to start each day with quite a bit of travel to get to any of the attractions downtown. (Probably about 45 minutes to 1 hour, counting driving and parking, or driving and CTA train to the city.)

I lived in Schaumburg for two years. It's not exactly a cultural mecca unless stripmalls count as culture. The Chi-town action is downtown and some of the near-downtown neighborhoods.

The downtown Ren is walking distance to a ton of stuff and a short cab ride to everything else. And it's a great hotel in and of itself.