I find myself largely in agreement with 10secondsurvey, with the caveat that I've never flown with JetBlue. That having been said, my decision process in selecting a flight is:
1. Is there a direct flight?
2. Which airport is used (e.g. always prefer Linate for Milan)
3. COST!
4. Service frequency - if I miss this one can I easily get another.
4. Airline preference (i.e. personal or anecdotal experience with reliability, punctuality and service standards).
There might be a 0. Safety, but this is only relevant in the third world (and even then it's very hard to make an informed choice).
So preference for my favourite airline(s) rarely plays a part in this process.