I certainly prefer to put up with a little inconsistency on AA, versus having a chance at a good seat on UA. If you're booking with plenty of advance purchase, you'll probably be fine on UA, but I like a bit more flexibility in my travel, like the ability to standby (preferably without a fee).

d

You don't need advance purchase to ensure E+ on UA. While it might happen once in a while, I have never seen E+ fill up before E- on UA.