There are a couple issues when reviews compare the GTX 460 versus the AMD 6850/6870. Most will compare the stock speed versions of the GTX 460, but the majority of GTX 460's on the market are factory overclocked versions. The other issue is the confirmed image quality hack that AMD used in their recent drivers which pump up the performance numbers at the expense of image quality.
(Reference: http://www.guru3d.com/article/exploring-ati-image-quality-optimizations/).
So if you compare a real-world GTX 460, one that comes factory overclocked with a nice custom cooler, with a comparable AMD card, the differences are minimal, and the purchase decision should come down to individualized tastes. As I am one who plays games that feature PhysX, for me the choice for an Nvidia card is obvious. It is also no question that the GTX 460 will perform DirectX 11 tesselation better than an AMD card, making the choice even more obvious for future-proofing your decision.
In terms of price/performance, you can easily find a nice GTX 460 for less money than a comparable AMD card. If you can find a MSI Hawk, an EVGA FTW, or Gigabyte Superoverclock you will be very happy.