Setting aside personal preference considerations, that’s an easy answer, go. You can afford a bump in the road and any reasonable estimation of a career at United versus JetBlue has less downside risk, and more Upside potential for United.

even if a big 3 buys jet blue you are unlikely to have some huge seniority windfall

Yes, it’s possible you could be furloughed briefly.
It is highly unlikely the nasty stuff from the lost decade repeats. The industry was undergoing a lot of structural changes that are now done and can’t be repeated. There are a lot retirements coming up over the next decade.

take the class