One interesting difference in upgrades is that UA uses direct origin to destination distance for determining the number of 500 mile segment upgrades required, while AA uses the total mileage for all segments. For example, if you fly A-B-C, UA will compute the mileage (and hence number of upgrades required) as if the flight were direct from A to C. AA will take the distance from A to B and add the distance from B to C. In many cases it doesn't make a difference, but in some instances AA will require more upgrades.

Purchasing upgrades is now less expensive on AA than on UA. UA charges $50 for a 500 mile upgrade ($200 for four); AA charges $45 each. However, if you buy 8 or more upgrades at a time AA only charges $31.25 each. This is a substantial difference if you need to buy upgrades. Also, AA's upgrades do not expire.