I've done the ORD to Madrid run a number of times. For me, it's all about not having to change planes, so I'd go w/ Iberia, whose service, food, etc. were all fine (and you do get miles on AA). UAL does not directly serve Madrid; you used to be able to go to Dulles and go on their partner SpanAir, but I don't know if that flight is still available.

I just think minimizing the plane changes makes it much more tolerable and also reduces the risk of irregular ops, weather, etc.