hi gumpy - that was my personal experience, I was quite underwhelmed & disappointed by the business section in the 747 I was in, perhaps it was old and needed an upgrade itself. I don't know what premium economy in that same 747 was like, I presume it would not have been anything like the A380. Possibly the only reason to go via LAX instead of SFR was to get on the A380. Now that Qantas has abandoned SFR for ?Dallas, no choice in the matter if you are heading to SFR.
If work or someone else is not paying for your airfare, my experience was that premium economy on the A380 was much better value for money than business on the 747 for the SYD-LAX/SFR leg. Obviously may not be the same for others.