Obviously you have your reasons, but for wanting the be ORD based and having prior Part 121 experience, I'm surprised you didn't go for SkyWest. They'll pay you year-for-year based on your previous 121 time. If you're coming here for the flow (which I suspect you might), the company doesn't exactly have the most honest metrics for prediction ... but I think you know that.

Good luck in class on Monday. If I were in your shoes, I'd take the 145 -- that way, when they displace you to 145 CA in the very near future, you can continue on one airplane and then bid into the 175 as a captain once a vacancy pops up.