Flythebus,
My choice would be United. AA has acquired TWA and I am sure you'll find pilot integration problems sometime soon.
While United is in talks to acquire USAirways, there is a good chance that it will not happen for antitrust concerns.
United has, in my opinion, a better fleet of airplanes than American - UAL operates 747-400s, 777s (the biggest fleet in the world), 757s/767s, A320s and B737s (with a few B727s hanging on). AA operates 777s, 757s/767s, A300-600s, 737-800s, FK100s and a boat-load of crappy MD-80s (various models).
When you add in United's new pay deal, it should be the obvious choice...
Of course, either one is better than neither at all.