T-Mobile coverage in Chicagoland is generally pretty good. And where there's a decent HSPA+ signal, you'll generally get faster data speeds than AT&T. But choosing an MVNO that uses AT&T's network could be a better choice if you travel a lot (especially outside of metros).
All that aside, no matter where you go in the States, you'll either be on AT&T or T-Mobile's network.
H20 Wireless: AT&T service
Red Pocket: AT&T service
Simple Mobile: T-Mobile service
Straight Talk: AT&T or T-Mobile service (you choose which SIM card you want when you sign up)
T-Mobile Monthly4G: T-Mobile service (obviously)
Both Simple and Straight Talk offer "unlimited' smartphone plans ($50 and $45 per month, respectively) that have "soft-capped" data. This means that they don't set a definitive limit, which could be an issue for data-heavy users. Rather, they reserve the right to terminate your data service if they think you're abusing it. For most people, this monthly cap falls in the 2-3GB range, but YMMV.
T-Mobile's Monthly4G has a few prepaid plans to choose from with different "4G" data allowances that are "hard-capped" and throttled thereafter. After you exceed your monthly cap, they simply throttle your data service down to 2G/EDGE speeds. I'm currently on their $60/month plan that hard caps at 2GB (enough for me).
MVNOs like H20 and Red Pocket in comparison, aren't very good values, IMHO because their $50 plans have pretty low data allowances and they charge for additional data a la carte. For those that must have AT&T service (for coverage reasons), Straight Talk is far and away the better value.
Hope that helps.