The quick version... it changes. Constantly. Flying shuffles back and forth between the 737 and the Airbus here, but the 737 is a gigantic system and getting even larger, so you can end up anywhere. Fewer redeyes than the west coast bases on the 737 but more legs per day since you're more central. Shorter trips go more senior, I think either narrowbody is just as likely.
As for the widebodies, the 777 base in ORD is closed. 787 is very senior, don't worry about it for a long time. 757/767 is mostly domestic with a few international departures, not that senior but holding a decent line on it, fairly senior. Days off and credit for any airplane.... very seniority based again. Junior lineholders might be 13-14 days off and 88 hrs of credit. Senior people getting 16, 17, 18 days off.
Premium trips, by the time you're on property, it's gonna vary again. Right now there's been more on the 737 then the 320, but that could all change in a blink.
None of your questions are really easily answerable, you'll have plenty of time on reserve if Chicago is your ultimate goal to figure out which airplane/QOL you want, and it won't really matter for your first 2-3 years of seat lock if you come here in an Airbus, a 737 or the 757.