The other thing to consider is that if there are any weather delays or ground control programs in place when departing CHI, Delta ops will often prioritize mainline flights over Connection flights, meaning if there are restricted take-off/landing slots, that Embraer flight might have a greater chance of being delayed or cancelled.

That being said, I wouldn't recommend booking a flight based on the chances or irregular operations occurring, and I find the passenger cabin of the E170/E175 to be more comfortable than the MD-90. Although for a short hop like CHI-MSP, I might still flip a coin and go with the closest airport and ease of getting from "door to door". In your case, I think that means leaning toward the MDW departure.