Your mom might consider doing a mileage challenge on AA and then getting status comped on UA. She can then choose which way she wants to go. The big pluses with UA are E+ as well as the easy redemption of miles. AA has cheaper 500 mile upgrades, but I've found them far more difficult to use.

A note about DC airports: IAD is not only one of America's worst airports, but is also located in the middle of nowhere. Unless your mom will be working in deepest darkest Virginia, she should fly into DCA whenever possible, even if it means making a connection. The other alternative is BWI, where one can catch a train from there straight into Union Station.
Good luck,

Bill