For me, a key consideration is who flies two aisle aircrafts on the routes I take. For a long time, United flew 777 on LAX- Miami (on one of the two daily flights), which made it my airline of choice on that route. Now it's 767, still pretty good. When it becomes two 757 or Airbuses, then I'll probably switch to AA, which has NS to Ft. Lauderdale with no Sat nite stay requirement for deep discounted coach.

As far as leg room, for me the issue is more often elbow room (width is my problem). So while AA has more legroom in the back, the fewer seats mean that you are more likely to have someone next to you, no?