Where will you be flying most?
- Who offers the most or best-timed flights to those destinations?
What kind of fare will you be flying?
- Full or discount? Delta doesn't offer upgrades on the lowest fares...
How much do you fly? Will you be achieving status with the airline? (Flying 25k/yr minimum generally)
What benefits in a FF program are most important to you?
- Generous upgrades? Continental offers 'em free domestically, but their int'l upgrade policy is pretty bad.
These sorts of questions will help tailor the answer...
I personally choose UA for most of my flying. I'm 8 mins from DCA from my office. Long-haul flights I'll generally drive out to Dulles to catch a non-stop. Otherwise I'll connect at ORD. I like the upgrades I get, guaranteed economy plus on the rare occasions when I don't, plus award redemption for F tickets to SYD...