I do not completly disagree with the above, but me thinks that in the most stated argument 340 vs 777 it's not only about no. of engines but also airframe/foil generally. I would conquer that a 777 with 4 engines, all about half the size of a GE90 or similar and half the FF (which should be achievable) would be as efficient as the present one, and thus more efficient than a 340.
The problem with Airbus in this instance lies in my view that they are generally heavier. Same comes to my mind when you compare 380 to 847-8F. But also very apparant 320 vs 737. Airbus puts more emphazise on solidity, technical perfection (sometimes perverted) and passenger comfort. 787 might be the first out of this series.
Dani