You're correct. The comfort level of the seats is pretty much the same.

The 789 obviously is the nicer/newer aircraft, though the 77Ws have recently been retrofitted. At the end of the day it doesn't make much of a difference IMO.

One other consideration: AC often sells upgrades (LMUs) to premium economy for ~$200-$250 on that route. PE is much nicer on the 789 than on the 77W, so if you*re potentially interested in paying for an upgrade, definitely choose the 789.