It depends how well you sleep on planes. If you are a good sleeper, take the night flight and sleep away as much of the flight as possible. If not, take the day flight and crash out in the evening in HKG. This is really the only way to avoid jet-lag. If you are not fully confident of sleeping well on board, the night flight could well entail being up for 48 hours straight (16hrs before, 16hrs during, 16hrs after the flight). Personally, I avoid overnight westbounds like the plague for this very reason.