Remember for most of us when you fly UA, you will be in a seat E+ with 36" seat pitch. That is so much better than AC with 31" in the front seats! [the ones they reserve for the BEST customers with less leg room.
here is the info from seat Guru on UA seats.
They typically fly midrange and transcontinental flights, but also fly to and from Hawaii. First Class has 38" pitch and 20.5" width. Economy has a 36" pitch for Economy Plus and 31" pitch for Economy, both with 17" width.

Upgrades with UA are much easier. Have a great flight!