Good question. From what I gathered at the Women in Aviation conference, talking with friends at those airlines, studying up on them and checking out this forum, I've come up with this (just my opinions):

FedEx:
Pros: possibly the pinnacle of aviation, great aircraft, great pay, steady business model, numerous internal recs and sponsors
Cons: poor domiciles requiring commuting, extreme high-quality and number of applicants, not hiring, lots of night flying, long upgrade time

United:
Pros: again, a pinnacle of aviation, great pay and benefits, great aircraft, great list of domiciles, lots of upcoming retirements
Cons: no internal recs, lots of high-quality applicants, not in the first, or maybe second, third or fourth round of possible interviewees

Alaska:
Pros: great domiciles, I can go home, great pay, already 737 typed, varied flying, sense of community
Cons: thousands of applicants for about 80 spots, no internal recs

Delta:
Pros: again top of aviation food chain, lots of internal recs, great pay, great aircraft, great domiciles
Cons: not hiring off the street until possibly 2015

JetBlue:
Pros: great aircraft, great domiciles, Latin America flying, young company, great corporate culture, couple of internal recs, THEY'RE HIRING!!
Cons: lower pay, possibility of being left out to dry with a merger

Overall, there's pros and cons to each airline, I could be more detailed about each, but forum posting aside, everyone I've talked to at JetBlue loves it, loves the flying, loves the work atmosphere, loves the people they work with. Sure, there's things that could be better, I've been told that as well, but for me, JetBlue would be a career airline.

Plus, I was very impressed by the pilot recruiters at the JetBlue booth. Didn't even get a chance to talk to United, Alaska had so little time, and I already knew nearly everyone at the FedEx booth.