I was hired by AMW during the 2000 rush...
Back then, upgrade was quick and usually did not cross aircraft. There were clauses in the contract that prevented a Jet FO to 1900 CA, most notably a seat lock.. Also the training contract that one must sign also stigmatized transitions by the senior guys until they had 4 years therefore avoiding the contract (this was when everyone was hiring!).
Since then, Mesa made the upgrade to the jet a little more difficult for the Dash and 1900 guys (additional requirements, letters of reference, personal interview as well as minimum times). All along Mesa was hiring like gang busters into the jet making the eventual upgrade even more competitive.
I have stayed in contact with several of my classmates and former captains. Most of my class mates (mid 2000) have either upgraded in the 1900 and then to the jet as a FO OR they went straight from 1900 FO to jet FO. Granted in just about every case it was almost 2 years before they got a jet FO position (and they are in the mid 700's in seniority). A few of my CA in the 1900 have made it to the jet but most have opted for the Dash as the lines are better.
During my time with the company... Usually the MAPD guys got the jet (automatic FO's for years!), 1500TT or better got the jet and the rest were divided up based on need to the Dash or 1900. Some MAPD classes got the 1900 but that was only during the shortage of 1900 drivers, again AMW was looking for guaranteed FO's for years. At the time when AMW was averaging upgrades less than 6 months Mesa did offer a 1900 to higher time guys who could upgrade immediately.
I agree with JETDRIVEN that the ability to hold a hard line is better in the 1900, but the QOL might me better in the jet. 1900's are based at small outstations and getting to and from can be a challenge (unless you live there). Most 1900 junior hard liners average about 10-12 days off and you will lose a few days in commuting. The Jet's are based at hubs and commuting is a lot easier, the 1900's based at hubs are almost all reserve unless your senior so it really is no better than a jet reserve position other than for the pay.
Don't know much about the Dash as it use to be mostly a western base but now with CLT I would guess JETDRIVEN could better answer that question...
My question to JETDRIVEN or any MESA or CCAIR pilot... Did JO move the Dash's to CLT to "remove" CCAIR or was it to squash PDT in CLT?