LAX-HKG is a slightly longer flight than SFO-HKG if you would like more time in Polaris.
LAX has a better Polaris Lounge than SFO, IMHO.
789 is quieter than 773, however Polaris seats on the 773 is a bit wider than those on the 789.
789 has higher humidity level than 773, so it is a bit more comfortable than the 773.
It is a toss up.