I've worked at both so can provide more insight then most. I'm currently at Mesa and while I have nothing negative to say about MESA, the choice in your situation is a given. PSA hands down. If nothing else the ability to not commute is huge and is the deciding factor. PSA has a better contract and a flow should you need it(6-8 years). Once you do put in that reserve time and hold a line, the SAP is amazing. It does not exist anywhere else in the regionals. Not at that level anyway. Since daddy American has deep pockets and is not cheap like Mesa, you also have much better tools in the cockpit. Ie PDC, D-Atis, georef on taxi charts, etc. As others mentioned you have much better priority when you are non revving. The only benefit I would say at Mesa is that management is more laid back. The guys at PSA have sticks up their as** and think their ****** doesn't stink. Note I said management, the pilots and crew are great to work with. As far as IAD you will get it right out of training if that's what you want. It is the most junior for CRJ's. You may be on reserve for a month or two(which is nothing) but if you are, they will use the crap out of you. I hear what you are saying regarding the 300K but there is also a lot to be said about quality of life. I was prior Navy as well and didn't think I would mind the commute but it wears on you. If you ignore my advice we can at least split the referral bonus for Mesa.
PM me if you have other questions. Happy to help.