Remember my Cardinal Rule: Join a program for any airline
you fly, even if you think you'll never fly them again. That said, you do need
to have fidelity to one or two "focus" airline programs or you won't ever
earn enough miles. Which one should be your focus? That mostly depends on you:
Where you live, where you are likely to travel on paid (miles-earning) tickets,
and where you want to travel on free tickets.
For example, United is great for Boston,
New York, or Washington
to Denver non-stops, with fairly good
connections elsewhere in the Rockies and West via one-change connections in Denver. But they are poor to
Florida from
the Northeast (you have to connect no matter where you're going.) They are good
to Europe and Asia. American Airlines is poor
to Denver (you must connect in Dallas or Chicago,
exceedingly busy and delay-prone airports) but they are good to Caribbean and to
Europe, etc. Delta is good to the south,
excellent to Salt Lake City,
with one-change connections to rest of West. They're also good to fly
north-south around the East Coast. I can't tell you which ones are right for
you, since your travel patterns and travel wish-list are likely different from
mine. But think about where you go and want to go, and then pick the program
that uses airlines that go there with convenient service. Since I live near Denver but often have some business in Boston,
I chose United as my primary focus program, for their convenient Denver service. I have
family in Florida,
so I chose Delta as my secondary focus program, since they have a good East
Coast schedule.
But wait, there's more! That means I also can use US
Airways for my United Miles, and Continental and Northwest for my Delta Miles.
I can use other airlines to earn miles, or use miles from one airline to fly on
another free. How? The magic of Airline Partnerships!