I had one occasion where I flew United this year, SNA-DEN. It was on a full Y fare ticket. One week before the flight, I had called United becuase my itinerary stated that seats would be assigned at check-in. I asked if they had any exit row seats. They said they could not pre-assign me an exit row seat, even though they said seats were available in the exit row! So I asked if I could get any window/aisle seat assigned, and again the answer was a resounding NO (including the return a week later).
Now, I understand that UA blocks certain seats for PE's, etc., but I was flying on a full fare ticket! If this isn't reason enough, I don't know what is. In past experiences I have had with UA, seating problems are tops (never been elite with UA, but always on full Y fare tickets). I really have not flown them enough to get a better feel for them. But seating is my biggest peave, and UAL has given me bad experiences.
Catman, AA gets a paws up from me (yes, I am elite, Exec Platinum) MOST of the time. There are times when I am particularly upset with certain services. Elite status does matter, as it seems to with United, and probably most other airlines.