I agree with the notion that your destinations make a difference.
Despite all the complaining on the UA forum here, I would characterize UA as an airline that is generally stable after several really bad post-merger years. Its starting to fix some of the little things that bothered people, and has improved the big things (such as on-time performance) as much as it ever will.
My impression of AA is that they didn't break all the little things like UA did, but a lot of the big things are trending downward. I mainly fly UA though.
If you travel internationally, I really like UA's partners in Star Alliance.
The real question is where will each be 2 years from now when you have dedicated your loyalty to one of them. My guess is they will look a lot alike.