I've flown a LOT of ORD-MKE and MKE-ORD flights. I have yet to have one of them either a)delay so much that I miss my connecting flight or not outright cancel.

For 3.5 hours I'd suggest driving. By the time you're finished with getting to the airport, going through security, waiting for the flight only to find it's late or cancelled, you could have been at your destination having a beer