I'll be the contrary guy.
Stay at Mesa. As for shtty regionals, I've been there, done that, so I know the sacrifice it entails. But there are clear upsides. Upgrade - moving to the left seat puts you on a high that lasts approximately three months, during which you don't feel the pain of working for idiots. A small thing, but being a CA is just more pleasant. The sooner you're in the left seat, the sooner you'll escape the regionals. Remember, your goal shouldn't be "Escape from Mesa." It should be "Escape from the nightmare that is the regional industry." QOL at the regional level is a farce. I'm sure most would say a senior Horizon FO maintains a stellar QOL. I could never be a seven year Dash FO, because I would have slit my wrists around year five. What's the freakin point of it? Unbreakable rule: At every regional you will work harder and longer for less money than you would at mainline.
Prompt advancement at XJT is not a sure thing. Continued 60% load factors on the branded flying and $100 oil could rapidly send a quarter of the fleet to the desert. Upgrade goes from 2-3 years to infinity overnight.
Mesa will no doubt continue to suck, which may work for you. Chronic understaffing drives up attrition, always. Lots of people, after one to many JAs, say "screw this" and walk. This is your ticket to the left seat. My airline shrank every year I was there, but maintained the quickest upgrade in the industry by virtue of conditions sucking so badly. Even if Mesa is forced to scale back modestly, progress will continue.
I'm a big proponent of present suffering in the name of future rewards. Seeing the worst of it now really makes one appreciate the next step. Suck it up and get out quickly - but don't get out now.
Truer words never spoken.