Somthing I don't think anyone's mentioning is award ticket availability, the ability to use your miles for free trips.
You've got the miles, but if you can't use them what good are they?
AA, UA: good award ticket availability esp domestically. NO SATURDAY STAY REQUIRED FOR LOW END AWARDS.
CO: poor award ticket availability, for either free trips or upgrades. And CO (along with US now) requires Sat stay for normal domestic award tix. This makes them at best worth about what a discount coach ticket is worth, perhaps $400.
Without this Sat night stay requirement, a FF award ticket can be worth $2000 or more domestically.
Quite a difference!