I had a number of transatlantic flights on UA 777's and 767's between 1998 - 2003, all in the back apart from one upgrade to Business Class due to my original flight being cancelled.
I must say, I couldn't fault any aspect of their service, either at check-in or in the air. In flight meals in both classes were excellent, significantly better than BA. On a Washington - Mexico City flight we got excellent Mexican food in more quantity then even I could eat, in economy class.
Regarding UA vs AA, UA win hands down for me on virtually every aspect. Some of the worst airline food I have ever suffered was served on AA. I ask you, a Cheeseburger masquerading as a meal on LHR - LAX!!