It all depends on what you're looking for. UA only flys 747-400's. One trip per day out of SFO and one per day out of LAX. The SFO trips tends to be less crowded, but also tend to have a less attentive flight crew, though on both the FA's tend to not be the most attentive. If you are a UA frequent flyer, this is a great run to build miles on.

HOWEVER - if you don't need to fly UA, and you're flying in economy, you'd be better off with Air New Zealand or QANTAS (it hurts me to say this, being a UA fan, but oh well). The fares will almost certainly be the same, and there is greater seat pitch, better food and better service.

I can't comment on the differences in Business b/c I've only flown United in Business, not on ANZ or QANTAS.

Another trick if you want to take UA is to fly to MEL not SYD. The LAX-MEL flight is a new one for UA, and is often very empty. Of course, you'd have to then fly from MEL-SYD.

Either way, Sydney is without a doubt the greatest and most beautiful city in the world, so you'll have a blast once you land