IMHO, this whole thread is academic without destinations.

There are some obvious deciding destinations, such as DEN (UA) and DFW (AA) and a host of places with subtle differences.

In my experience, AA flights tend to be more likely to be on time than UA flights, but that could just be my luck.

With AA and the oneWorld alliance, you get one North American airline: AA. With UA and the * Alliance, you get three North American airlines: UA, US and AC.

Also, keep in mind the recent US-HP merger which has added even more * Alliance routes on which you'll get Mileage Plus EQMs.

Good luck on the new job and have fun getting those miles!

Edited to add:

Of course I forgot non-alliance partners, of which AA has (marginally) more than UA.

AA:
Alaska (non-stop service to SEA and ANC from ORD)
Hawaiian

UA:
Aloha