Expressjet is a great company. I worked there for a little while. Awesome contract and great people. I wouldn't be surprised though if they stop hiring soon. They are still hoping to get a new agreement with another carrier and if that doesn't happen they will have a whole bunch of airplanes that they say they will fly their own brand with. That's a very risky proposition - going it along with regional jets.
American Eagle seems like a very stable place. The problem initially is you won't know your equipment or available domiciles til after you start, but if AA starts recalling soon those upgrade times could shorten considerably. Remember they didn't hire for a long time so there is a big gap between the guys who've waited 6-7 years for an upgrade and the recent spurt of hiring over the last couple of years. I think if they had a domicile that you liked than it would be a good place to be with an eye to a stable future and getting based where you want eventually. YOu just have to be prepared to commute to San Juan for a while if that's where you end up. (Or move there).
I think Comair is in a very precarious position right now. There are a lot of people bidding on their DL flying and we've already seen DL sell off ASA. Who knows. For a new guy starting out in 121 though any of these places could be a good place to start out and remember - if you don't like it keep looking for something better. In the meantime you are gaining valuable experience to help you land the job you really want. Just my opinion.