Take the job ...
"A bird in the hand is worth two in the bush."
"It's always easier to find a job when you have a job."
Both tired cliches my dad gave me when I was younger, but there's a lot to them. Take the job, get the 121 turbine experience. If you don't like it, you can always look around for a better job and move on when an opportunity arises. At least in the meantime, you'll be earning a living (sort of).
To be pessimistic, say you go, then you get sacked as a result of some future development (J4J or otherwise). You'll be right back where you are now, except that you'll have some experience to add to the resume, still making you more attractive to other potential employers.
There are no guarantees in life, and especially in aviation. Thirty years ago, nobody thought Eastern, Braniff, Pan Am, or TWA would be T-U.
In any case, good luck and fly safe ...
R