I've never talked to an insider at Republic but from an outsider looking in I think they just seem to have so many unknowns and different operations mixed together that what little comfort you can expect from a regional may not be found there.
Now I'm sure being at SkyWest will taint my view but I think SkyWest is the obvious first choice, then ExpressJet as my alternate. Sure they may seem to be moving toward similar contracts and work rules but if push comes to shove my gut says Inc is more committed to SkyWest maintaining the status quo if not benefitting from future opportunities. If it can bring ExpressJet along for the ride, then great, but if not...
This is an oversimplified view and glosses over the many great things at each company as well as the negatives but again it's my gut feeling. I wonder if the original poster has made their decision? If so, who did you pick? We're you offered an opportunity at all three? If so, congratulations. I hope you are excited for your future. Know that your dues paying will make you jaded at times, but I'm still glad I switched over to aviation. I'm enjoying the process, just wish I didn't have so much debt.