TDK,
As a former JetBluer (now at NJA), I will give you my perspective on the matter.
First off, JetBlue is a great place to work. The crews are fun and you will enjoy going to work (well unless there's an IROP).
I think a lot will depend on your personal situation (duh).
For me, as the sole provider for a family of four, I couldn't pass up an offer from NJA once the new contract came around. My take home pay is significantly higher now, even more so when you factor in free medical/dental and no commuting expenses. Be sure to compare your current medical plan with JetBlue's and factor that into the financial picture. If my wife worked and had good benefits, that may have changed my decision.
As far as #4, it depends. I think most guys there will tell you it will be a long long time before you see the left seat of the Bus.
As it stands now, the Bus growth rate is basically nil (always subject to change) and with no significant retirements for 15-20 years, you can do the math. It will easily be 10+ years before you would be a line holding Captain. If you want to be the junior reserve Capt at JFK that may decrease the time slightly. I don't know about the 190 (obviously it will be less). Having said that, the upgrade time was not a significant factor in my decision (it's going to be a long time at NJA as well).
Nobody knows what the future holds. If (when?) JBPA gets up and running, how will that change things? In the short term, I definitely made the right decision to leave JB, but ask me again in 10 years and I may have a different answer (I hope not...

).