Hi Tom, I was in your boat this past January. I spoke with both and ultimately went with F9. If it helps, here were my considerations:

1) Both F9 and NKS are hiring about the same number of pilots per month, but NKS is about 50% bigger. Simple math says F9 is going to be a faster upgrade. (but they are both still WAY faster than legacies).

2) Though the airline industry as a whole is growing at the moment, the LCC's are operating more profitably and at cheaper margins, thus making them more resistant to market shocks... meaning I believe the chance of furlough is less there.

3) Despite what many may wish, the ULCC trend will, I believe, continue to take market share from the legacy carriers' domestic operations. As example, look at the European distribution of seat-sales between ULCC and their older names in the business.

4) NKS has a much better contract than F9, but I expect the latter to have a very significant compensation adjustment in their next contract (starts March 2016).

5) The pilot group at NKS and F9 is much younger and less bitter than... other larger carriers.

6) Pay is great, up to a certain dollar value. Then that "next dollar" of pay is worth less than a bit more quality of life. Find what makes you happy and choose from there.

7) Finally- and this is in hind-sight- I only had to sit reserve two months at F9 before getting a line. I'm not sure what it is at the legacies, but I imagine that's hard to beat.

This is just my opinion- hope it helps. Good luck to you.

Oh, and don't mind all the trolls here. Once you've "made it" to our career destination, sometimes there's nothing to do but complain. Some people look at the down side of everything. Stay positive