Thought: flying to LAx on a Sunday MAY be more prime to some business pax as they get in Sun afternoon and can refresh before a week of meetings or whatever, but if they want to maximise their weekend time with family(or whatever) then the later flight would probably be the one they'd go for which means J may be more full on that particular day. Further, the 380 sees chances of some J pax getting upgraded to F, with a resultant shuffle again opening up options.
The con to that is that the 380's have had poor reliability dispatch wise over recent months with the fleet absolutely stretched to the limit so if time is at all critical here, then the 744 may be the way to go. Of course if one books the 380 and it's delayed or cancelled there's further rebooking opportunities later in the day onto the 744 or via SYD.
conclusion? if it were me I'd probably go for the 380 but it's a very individual and unique situation very time and date variable.