747-800 vs 777-300ER?

the difference with the 787 over current models will be from 5%-15% its impossible to tell right now, but thats what computer models indicate. The 747-800 and 777-300ER are similar in range but the 747 is more suited for the trans-pacific runs over to asia because asia is more densely populated, more people on an airplane. The 777, is more suited for the Atlantic runs to Europe because it has fewer seats and is slightly more economical... even though fuel consumption differs slightly, airport costs are much less on a 777 than a 747.

The 747-8 will be using the same engines the 787 will be using and the 747-8 will seat around 450 passengers. Both aircraft should be able to fly from the US to Asia without any problems but I think the 747-8 should be able to seat more passengers. They say the 747-8 should be at least 10% -20% more efficient than the 747-400 model. As for the how much more efficient the 747-8 will be over the 777-300ER, I think it may be about 5% - 10% because even though it will be using more fuel efficient engines, the 747-8 does require 4 engines while the 777-300ER only uses 2.

Generally you need really long legs to realize the advantage of a 74. It is the faster airplane of the two.

If I were running an overwater airline I would reserve the 74 for the Pacific. I would use the 77 on Atlantic runs and probably use something a little smaller on domestic routes.