Both are awesome cards. If you are not going to be overclocking, 5850 is going to be a lot faster, especially at higher resolutions with AA.
If you don't mind overclocking, for $230 you can get a card that at 850 mhz will equal the performance of the 5850, while being the
quietest videocard of this generation. If you can find a 5850 for $260, then it's also an awesome deal as it can be overclocked to 5870 speeds (of course at $260 I'd pick the 5850). However, most 5850s are now hovering around $290-$300. In that case, I'd rather take the GTX460 and pocket $70 towards a next generation $200 gem

(and you know you will want a new videocard in 12-15 months from now!), while enjoying a very quiet videocard at load.