I don't do that particular commute, but at a glance, it looks like BWI would be a better option because there are more flights per day, increasing the odds of one lining up closer to your scheduled trip. (You also have three flights a day from ROC, which might be another option.)

The downside of BWI is that there isn't a cheap and easy way to commute into IAD or DCA as a backup and get to BWI. MDW, on the other hand, is a ~95-minute train ride to ORD, and that would add several United and American flights as commute options if the timing works out. Check out FlightLookup Display Flights as a tool to see various commute options, and plan on working mostly weekends initially. (So look at schedules for Friday and Saturday going out.)

I commute to OAK and am currently on reserve, and have gone to SFO several times because it let me sleep in a couple hours longer before leaving my house. As long as you're phone contactable at the beginning if your reserve block, you don't need to be anywhere near your base; you just have to be able to report within 2 hours of the call. SFO-OAK is a little over an hour on the train, so that works out very nicely for me. ORD/MDW would be similar.