5850 is around 10-15% faster than GTX 460. Different Manufacturers make different card models (logical) with different cooling and/or clock speeds. I can't really say, which card is better, they are both good.
They are both 5850 card at Tiger and CanadaComputers, but didn't manuf. and some of the numbers of the card is different - so I find it confusing.
Tiger Direct : XFX HD585XZAFC Radeon HD 5850 Video Card - 1GB GDDR5, PCI-Express 2.0, CrossFireX Ready, Dual DVI, Display Port, HDMI Includes Double Lifetime Warranty
CC : Asus EAH5850 DirectCU/2DIS/1GD5 ATI Radeon HD 5850 Chipset (725MHz ) 1GB (4Ghz) GDDR5 Dual Display DVI/HDMI/Display Port PCI-Express 2.1 Graphics Card
Both GDDR5, both 1 GB, the only different I see if PCI express and CrossfireX ready - the PCI express is depending on the MOBO I believe, but CrossfireX ready I am unfamiliar with and not sure if it is worth the extra $10.