Just some general advice:

The pay does suck everywhere, but objectively more so at Mesa. If you've run the numbers on all the carriers and can be financially responsible on Mesa pay, then no worries.

Recruiters can and do say a lot of half truths to get you in the door. For example, if an applicant says, "Can I request IAD as a base?", the recruiter might say, "Sure! I've got you marked down for IAD!" That sounds nice, but realize they have literally nothing to do with it, and they only have, at best, an educated guess of where you'll end up. Bases available are determined by operational needs, sometimes right as your class starts. Base awards are given out by age, oldest in your class first.

Your base/equipment normally has NOTHING to do with upgrade time. The only exception I can think of is if you can hold a line faster at base ABC rather than base XYZ, which lets you get your SIC a little faster. That's splitting hairs and should not be a part of your decision about upgrade time.

The same is true of job security. Your base might close, and that might suck, but that's not the same as job security... unless your base closes AND you get furloughed.

I've commuted across the country, and I'm about to do it again. It sucks. It takes a lot of planning and a lot of energy. You will lose days off, either before or after your reserve days or trips. Sometimes you'll lose days off on both sides of a trip. My 2c is that commuting is only worth it if you have a solid plan to not commute soon, like moving to your base, or getting rebased to where you live. Some say never ever commute. Some have commuted for decades. Try it for a year and see what you think.

Edit: Also, I know getting an offer from a 121 is exciting, but make sure you apply and interview at a few others before coming to Mesa. Its worth having options.