Hi -
I have taken the LAX-HKG flight several times. Flight time 15.5 hours.
F/J - take the overnight flight. Have a fast meal, then grab a good sleep. Hit the ground running in the AM. Before you leave the airport, stop by the Arrivals Lounge for a shower and a change of clothes.
PE/Y - take the day flight. Get a good night's sleep at your hotel and hit the ground running the next day.