Ghilis - DL's an excellent airline and the current 800 lbs gorilla. Can't go wrong there. But if Brand X is your #1 choice keep applying. Back in the mid 1980's it wasn't uncommon for guys to have multiple choices OR work for several majors.

If Brand X isn't hiring, or is hiring slowly, the crossover point of staying vs. going gets extended and could be several years. If Brand X is hiring significant numbers the decision point, "I'm better off here now", comes quicker.