I would chose the one that gives me an interview and I manage to get an offer from, that being said, Frontier is just starting to grow where as Spirit has been growing for awhile. Upgrades may be faster here at F9 for people being hired today, at least it would seem that way for now, but the pay is behind Spirit. It will get fix but not tomorrow. Finally, look at the bases, where would you like to live. I recommend to live where you can avoid commuting which typically means better QOL. Honestly, I think it's everyone's guess as to which one will be better for a new hire. This industry is crazy.