I don't think it's that easy, Operaations....
1. How senior is the FL FA? Does the FA have pretty good control of their schedule, know what they can/can't hold?
2. How long is reserve projected at WN? Yes, I know they are growing, and it won't be long. But does the person want to go thru that all over again?
3. Same with payscale. Since you'll have to start at the bottom again, how much of a pay hit is it? Vacation time? Are you willing to forgo paid vacations for a year?
Unless I'm mistaken, all FL crews are ATL based. WN doesn't have a crew base in ATL, nor do they serve it. Commuting might be an issue, unless you want to move. Does WN tell you which base you'll be in before you start? Those ATL to OAK commutes might be difficult.
FL still has the bulk of their 737 order in the pipeline, plus the whole MDW growth plans. WN had a long road to get to their current contract, and in many ways, the "old" WN is gone. I'd probably stay at FL.