CHQ flys the 145 and CRJ-200
RW flys the 170
S5 flys the 170

If you want Chicago, you'll have to get hired on the S5 170. I believe ORD is fairly senior and shrinking as ATL stands up. Chances of getting hired to any certificate with 1000/100 are outstanding. Chances of getting exactly what you want are still good, but certainly not guranteed.