Originally Posted by Kacee

Depends on the aircraft, as they have different seating configurations. In general, AC has high-densified most of its long-haul fleet, whereas UA has only partially done that.

Depending on where you're starting/ending, there's also the issue of transiting another country. It's an added hassle, especially if you're Canada based, because the US transfer is a pita (Canada no picnic either).