... and I'm still not convinced 4 x GENx will burn less than 2 x GE90. That would as good as make the 777F obsolete, whereas in reality it's selling pretty damn good.
In theory a quad will be more efficient in long haul operations since the engines can be sized for cruise rather than the engine failure/takeoff scenario. The A330/A340 and 777 have certainly muddied the case, but it is a mistake to assume that a twin is inherently more fuel efficient than a quad. It will be interesting to see what the final 747-8 numbers look like.
And it is also a mistake to seek a one airplane wins answer. Freight is complex, and different airplanes are going to be better fits for different routes, markets and operators. Some operators are clearly very happy with the 777, some appear to not be. And I work for one that took a very, very hard look at the 777F and then signed for the 747-8.