Personal opinion is Spirit looks better than Frontier. They have some international ops (how many more domestic only carriers do we need...) and are making efforts to redo their brand and image. Sure the big three are loosing pilots due to retirements but what NEW ROUTES do they have ? More flights to Vegas ? What is "different" about one to the other, an important element if the economy goes south.

I think Spirit (and yes, Frontier) are good places to hang out while you wait for Delta to call. If they never call, you are a captain soon in 5 yrs or less and prob 200k+ flying new equipment.

Remember, in 1990, SWA was a "low cost" carrier that "only" offered peanuts. Imagine the SWA FO in 1990 who quit because TWA called him.