Objective look from the outside (but with a fair amount of knowledge of the regional situation):
Contractually, XJT is a much better place to be. Mesa's CBA is weak in many areas where as ExpressJet has a very mature contract that is pretty well locked down. Of course, your contract is only as good as the enforcement of it, and while I don't know specifics, from what I've heard over the years, the level of enforcement at XJT is decreasing but probably still ahead of where Mesa is.
Unless you are planning making a career at either one of these places (and I'd say that neither one will be around long enough for that to happen) the pay differential is negligible. You are going to make next to nothing your first year and then it will be survivable after that as long as you are financially smart.
Historically, Mesa had a pretty poor culture. Their CEO is an ass (you can TOTALLY say ass on here!) and beat the hell out of them. However, over the last few years, necessity has driven them to make some changes and most of the Mesa pilots I've talked to recently have said it's better than it's ever been. Not that it's great, but that for the first time in a long time (or ever) they are feeling slightly more upbeat about things. XJT on the other hand was, the golden child of Continental until they were divested and although that glow followed them for a number of years. Over time it's kind of faded and then through being bought by Skywest and forced into a shotgun marriage with ASA, it's pretty much gone.
Mesa is growing a fleet of "large" RJs. ExpressJet has a ton of 50 seat RJs and no real plans for fleet replacement right now.
Bases... You'd certainly be able to hold Phoenix before Denver, however I don't know how long that is taking right now. Last I heard (and this info is about a year old) it was 2 years to get west with them.