Once upon a time I was attending some arcane finance class in college, in an attempt to learn some of the in's-and-outs of the business side of this business, when the discussion turned to a certain Texas-based airline. At the time, even though they had started as basically a regional airline, they had grown to some longer routes, including several international destinations. They were probably the healthiest airline of the era, at least on paper. They were known for sound, if sometimes unconventional business practices, the flamboyance of one of their founding members, and their ability to make a profit, even when others were drowning in debt. They were also known for the distinctive, and by some accounts, downright ugly livery of their fleet. Among would-be pilots, they were known for being a "no-a$$hole" airline. You had to be recommended by somebody, ideally several "somebody's," to even get an interview there. They were "Good Old Boys," the darlings of Wall Street, as well as the darlings of the industry.
"That's where I want to work!" I thought to myself, as I started contemplating how I would cultivate the connections I'd need to get a recommendation there.
The year was 1976, the airline of course was Braniff, and less than 6 years later, they were bankrupt.
If you were to poll every pilot flying for a major airline right now, you would probably find that half of them would gladly trade their seniority at their present carrier for the same date-of-hire at one of the "Flavor of the Week" carriers that are so often mentioned on this board. That wasn't true five years ago, and it probably won't be true 5 years from now. The question before you is not "who's doing well today?," it's "who will be doing well 5-10-15 years from now" Offhand, I can't think of one single airline in recent times, that has been an "A-list" employment opportunity for more than about 5-8 years.
It's a cr@p shoot at best. Whichever one you go with, I wish you the best of luck.