Thanks to all. The thing is, on this trip I need to use miles rather than accrue, so it's really a case of doing it in the most efficient way. Plus I want to do as much of it as possible in business class. I get the impression that QF at 105,000 is a better business class product than UA at 90,000.

BTW until reading the replies I'd never heard of SWU (System Wide Upgrades?) etc. Is this because I'm only Premier on UA? How do you get them? And can I use AA or QF miles to upgrade from a Y ticket?