EJECT!! EJECT!! EJECT!!
First off - go for the NOW class in the Brasilia...why? Seniority is everything. Those 40 numbers could mean the difference between being on the street and flying a jet at a later date.
I don't know what your flying background is like but I'd probably assume you're a low time pilot. If so, you could probably use some "real" flying before you get back to a CRJ. The CRJ is cake. Besides..you don't want to be a "Glass Baby" do you? Go make some noise and enjoy "BETA"!
Secondly - If I remember correctly, you have to finish training in order to be bound by the Mesa training contract - no? If you don't start the SIM, then there really isn't any true loss by Mesa. I think you might get a "get out of jail" card in that scenario.