nux, you are correct that Qantas HKG to SYD is a 380 on Fri - Mon, and a 747 the other days.
However I was planning to do the SYD to SIN on Qantas (with HKG to SYD on Virgin). There are two flights a day on this route, one is the 330 the other 747. From what you are saying, the 747 is likely to be the better option as this has more chance of the mkII seat.
The reason I'd made this choice was for a number of reasons:
1) I have exactly the right number of VS miles for 3 x oneway HKG to SYD in upperclass.
2) Availability on Qantas appears tighter for HKG - SYD
3) If I book SYD - SIN on Qantas, I can switch it to BA if availability opens up, saving me 40k avios (due to the 241)