Largely the same shtick with regard to dedicated check-in and priority boarding (though with AA you board with first class not just in a higher group). At platinum there are complimentary domestic upgrades and you earn "500 mile" upgrades as you fly. I think I have 20 of them at the moment. No economy plus but I have only missed an upgrade once on an early-morning flight from DFW to PHL. The food in first class is better in AA than other domestic first class cabins I've experienced (US Air is particularly bad; I've never flown first on an Asian or Middle Eastern airline so I obviously can't compare). And 100% bonus miles too.
Out of Chicago I'd probably go with United. But AA ain't a bad deal.