If you're IN Chigago, you'll probably want Verizon, not SprintPCS. (Better network.) Not sure about the rest of "Chicagoland" though.
Let me add: If T-Mobile works well in the areas your family needs it, keep it (if their family plans are to your liking). And within areas that Tmobile has service, the quality of service is generally better than Sprint. So, don't switch to Sprint unless you're a glutton for punishment. If you're going to switch with the goal of better service/more areas covered than T-Mobile can offer, look to Verizon.
EDIT: Oh damn! I misread the original question--for some reason, I thought it was between SPRINT, T-mobile, and Verizon.
Nevermind this non-sense. I don't actually know anything about Cingular.