First off, the A380 does not have a 15-20% better operating cost than the 747XS. Possibly the 747-400, but until the A380 is winging its way across the Pacific we truly won't know.
I would think that United would go with the 747X. They do not require the extra amount of capacity that the A380 offers. But, as others have mentioned, SQ, VS and QF have opted to order the A380. If you look at those airlines though, you will see that they are highly hub oriented and truly operate out of only 1-2 airports as their main base. Like most American based carriers, United is more fragmented. They have a large presense at ORD, SFO, JFK. Along with Dulles or is it Reagan? What I am trying to say is that United is a different kind of airline than Singapore or Virgin. Virgin operates on a system of 1-2 flights a day and needs capacity. SIA does also. United is a little bit different. Notice that VS operates a 747-400 on its LHR-EWR route while United operates a 777-200ER.
F.Y.I It has been stated time and time again that the JAL and ANA are looking to see what UAL and NWA are doing in the "Trans-Pacific" wars. It seems that UAL has made an unofficial decision towards the 777-200LR. I would look to see the 777-200ER begin flying more trans-pacific routes. Also, not to start an A380 v. fragmentation war, but it used to be UA, NW, JAL and ANA flying 744s across the Pacific. Now,
JAL: 747-300/400, MD-11
ANA: 747-400, 777-200ER
UAL: 747-400, 777-200ER
Look closely at what the new guys are flying. AA and CO both operating the 777 not the 747 and certainly not the A380. In fact, ANA switched their NRT-ORD service from a 747-400 to a 777-200ER. IMHO you are starting to see the fragmentation of the Pacific. Now, that does not mean that 747Xs and A380s won't be needed, but they will only be need on the trunk routes.
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