Depends on what you're looking for:
Bases - Republic has A LOT of them and it's mix between hub's and outstations. If you want to live at one of these bases, that's an added bonus. The problem is that we're notorious for closing down bases (reference FLL and MCO). XJT has fewer bases but is less likely to shut one down (I'd be wary of ONT or LAX until they're established).
Pay - On the FO side XJT wins, but on the CA side all things are fairly equal. FO longevity and amount seems to be a sore spot for a lot of people here and they want it address on the next contract. Our's is up in October, don't know when XJT's is.
Upgrade - From what I understand, this is equal. We're both running around 2 years. This could change overnight though.
Contract - Here's where XJTer's may not agree, but from the people I've talked to and with my
limited understanding of the XJT contract, we're both fairly equal in the intent of our work rules. Republic has no junior manning, XJT does. All our days off are "hard" days off, they can't touch you unless you get stuck somewhere due to WX or MX. If you do get pushed into a day off, you get 4 extra hours of pay or another day off, your choice. It's 11 days off for reserve, 12 days off for lineholders. The problem with the Republic contract is that since we're "newer", it hasn't "firmed" up yet. Things we
intended to go our way were not worded as well as we wanted them to be. But, with a new contract up in October, they will get addressed (hopefully).
Reserve Period - Depends on where you're willing to be based. There were people in my class that never sat a day of reserve and it seems like that trend would continue. However, if you land the 170 on the Shuttle America side...you could have a wait involved. CHQ has by far been growing the most on the 145 and CRJ side. This is where any reserve period would be the shortest.
Equipment - I wouldn't recommend ANYONE come to Republic for the 170. It has engines under the wings and holds 70 people, great. Despite what I personally think about regionals carrying that many people, no one asked my opinion when they started flying them. I'm not totally happy that FO's get paid the same on either equipment. If you got a chance to choose your equipment (I kinda did) I'd go 145. It's more fun anyways
Stability - Both are profitable companies. XJT may have some uncertainty with their branded flying, but with the Delta contract outta LAX they are showing they can diversify and spread their risk around. This is a good thing. Republic is going through some growing pains. We picked up so much flying so quickly that we are trying to catch up with ourselves. I won't speculate on the future, but somethings are going to hurt while we get settled into our new contracts with CAL and Frontier.
Pilot Group - The only thing I know of the XJT is from what my close friends their tell me. They are all happy enough with the people they work with. Haven't heard any complaints. The people I've worked with at Republic have been awesome. Great group to putt around the sky with and they will teach you a lot. I got really fortunate to fly with some pretty senior CA's fresh outta training and the experience was invaluable. They're also fun to hang out with and look out for each other. I'm sure XJT is the same. It seems like both companies are pretty "happy"
Training - Another one that XJTer's may not agree with me on, but I think it's pretty equal. XJTer's do it in house with their own instructors, which works out great for them. Republic uses FlightSafety for their training. Talking to a friend who was in training with XJT, he said some of it got a little disconnected and they started just jumping around random stuff in systems. I've never trained at XJT so I can't say anything about it. But a company with 2,000+ pilots has got to have it's training program down pretty well, so despite any complaints anyone hears, I'm sure it's a good program. I really enjoyed the FlightSafety program. It was very well structured, laid back, and the instructors taught you what you needed to know to move on to the next step (Checkride and IOE). I didn't have any problems in either of them except for the stuff they tell you "You'll learn this on the line". Both companies share hotel rooms while in training. I don't know how XJT pays their people in training, but we pay $400 a week for the first 4 weeks then minimum guarantee after that. Since they consider that $400 per diem, it's not taxed so it actually comes out that you make MORE in the first four weeks than you would with just min. guarantee. Max per diem a company can pay per day is $60...the $400 comes out to like $57 a day.
So...basically, you need to decide for yourself. Hopefully an XJTer can give you some more insight into operations over there. CHQ was my number one choice and since I loved my previous job, I had the ability to sit around and wait for them. If they said no, I would have moved on to XJT. If you want any more additional info, feel free to PM me.