I went to see it today. An emotional rollercoaster, and well worth seeing.
But I found Flight 93 to be a better film overall. There are two chief differences between the two films.
First, Flight 93 shows the people at the other end of the phone. We get to see a wife’s or mother’s reaction to their loved ones calling them. There are protracted heart wrenching conversations between families, during the course of the hijacking. This makes Flight 93 a far more emotional experience. Much of the film is devoted to these phone calls.
Second, Flight 93 showed very little military reaction. The film instead centered on the passengers and the hijackers. No scenes of the planes hitting the WTC, only word coming through on the phones. The early parts of United are almost entirely devoted to air traffic control and NORAD reactions.
Two different approaches to tell the same overall story, I found Flight 93 to be the better of the two.