Diffferences that I have noticed between AA and UA:

1. AA allows stopovers on Domestic awards. If you were going home you could do something like ORD-IAH-ORD-NYC. Booking the return to NYC with a stopover at ORD would give you a free one way to NYC, then only have to buy a 1 way for the return. Since you can change dates, you'd basically have a free 1 way to NYC good for a year.

2. AA allows using partners along with AA on international awards with exception of BA. For example if going to Italy, you could use LH to go but then would also have to find availability returning on LH. With AA if you took AA to Italy, but no return was available on AA, you could also use SR,air lingus, IB or Finnair. This allows for a lot more flexibility which helps you get the dates that you need.

3. AA partner awards are good for all of Europe not just home country. LH award from UA to Italy would cost 50K to germany and then 20K additional from Germany to Italy. For cities UA doesn't fly to outside of a carriers home country (florence for example)minimum mileage would be 70K. AA carriers will do this on one awards for as little as 40K depending on season.

4.UA doesn't allow domestic connecting travel on international award during domestic blackout days. Wanted to return from Europe on Sunday after thanksgiving. UA wouldn't give me ORD-IAH segment because it was a blackout day. AA said it's allright since I'm using a Europe award and that day wasn't blacked out for Europe. So for awards requiring domestic coonection, you would be subject to both domestic and international blackout days with UA.

5. AA just seems to have better award availability.