You didn't say how tall you are - not usually a factor in FT questions, but it is in this case. If you expect to spend most of your time in the back of the plane and you're over medium height, I'd say UA for E+. Other than that, I'd go with AA on balance, though there isn't that much of a difference.

(While it's true that you'll usually have to connect at a European hub to get to most anywhere with AA, you probably will with UA most of the time too. Unless your usual destination happens to be in a UA city that AA doesn't serve, I don't think a wider choice of hubs to fly through is a huge deal. How many does one need?)