Both MDW and ORD are on the "L", elevated train, so it is very convenient (and cheap) to get into Chicago (about $2). You can buy a 3 day pass at the airport stations for example for about $12 or something in that range and get unlimited train and bus rides which includes to/from the airport.

Chicago has about the cheapest mass tranist system that I know of. Chicago is a great town in the summer. (but too cold in the winter).

MDW is about 30 minutes to the Loop and ORD is about 45 minutes to the Loop. But the trains have been running slow with track repairs. But if you are going to North part of Chicago - ORD is closer and it is quicker to transfer off the train and take the bus East toward the Lake, rather go downtown and transfer on the L to go back North.