Well im going to be getting the GTX460 soon to play Crysis, i can play crysis ok on a 9800 gt graphics card. so in my opinion both are way capable of running thiese games. however the ATI Radeon HD 6850 is rated at number 10 on Passmark whereas the "GTX" (not ATX admittedly) is rated at 22 !
So id say the first card would be best if your going by benchmark results.
well i personally have a gigabyte 6850 and it is absolutely amazing i can play every game on my computer including crysis at maximum settings at 1600 X 900 with an AMD athlon II x3 at 3.0 GHz
so i would go with the 6850 extremely fast card none of my games even posed a challange