If you can at all manage the extra time, I would suggest flying in as early as you can to Sydney. Sydney, at least to me, is really a great city to visit and warrants more than a day to visit the primary sites.
BTW - To me, the Blue Mountains would not be at the top of my list if I only had one extra day in the city. The good news is that this site has lots of people from Sydney and Australia generally to give you ideas of what to see and do.
The one edge I would give to UA and/or DL over QF would be the availability of extended legroom seats for purchase in economy. I think the seat pitch in QF economy is still 32", but if you are tall this is still no fun as compared to getting something more like 34" of seat pitch.