In my limited experience with AA, I found their customer service is leaps and bounds better than UA. Just calling AA without any status, I was connected to a friendly, American-sounding, very helpful agent, who helped me find some award tickets I wanted. Very quick, <10 seconds hold time... Try doing that as a GM at United.

That being said, if you have status at United, they treat you OK. And with CO coming into the Alliance, you could probably credit much of your flights to CO if you wanted to.

Upgrades on United are OK as a 1K. I have had good success this year, but I assume it is because others are flying less and cheaper buckets now than in previous years.

E+ is really nice I think. I have no experience redeeming awards on other Alliances, but I find it difficult to redeem using UA or AC miles. I did get some good Air New Zealand tickets this year from UA awards, but from the reactions of others on the board, I think I got lucky. Like upgrading, I think it might have something to do with the down economy.

If I had to choose, I would take UA over AA, because of CO coming into the Alliance should hopefully keep me off other airlines/alliances.