UA stays pretty full out of ATL to all their hubs. DAL has like 10 flights a day to ORD and maybe 5 to MDW. They are slammed some days, wide open on others. M/F are the worst. Lots of commuters on this route. Not sure what SWA is doing these days, but I do know they have a few direct flights to MDW. Also consider if any weather hits Chicago or Atlanta, you're going to be SOL. Especially winter weather in ATL.