It depends
Remember that the L stop is on the Blue Line and the ORD stop is a good 5 minute walk from the security gates at ORD. The Blue Line is considerably west of Michigan avenue. Absent a significant traffic jam, the cab will have you at the airport security gates faster. Sometimes the blue line stops for 10 or 15 minutes just for no reason you can tell. I have done both and absent financial considerations, a cab is easier and generally faster. Oh, I have lived here since 1972. I am not reading this out of a guidebook.