Firingsquad back Anandtech's finding: the 5850 mostly beat the 470 in benchmarks where the 5870 bests the 480 (minority of the time). In some of those cases, the 5850 even beats the 480, it's fair to say that the 58xx is better suited than the 4xx (I do not believe it's always a case of intentional bias). Hardware Canucks also back that notion. So I wouldn't say it's just Anandtech with that conclusion.
As for drivers, I tend to think that they are about equal nowadays, but it's certainly not always been the case (incidentally, nVidia 'broke' support for one of two of older game I have so I may well lean in favour of AMD from that perspective). Still, I can only speak for Windows based PCs, nVidia had the edge under Linux for a long time, and if that's changed then I would say that it's in the relatively near past).