The 6850 is the better card of the two, however, neither card is going to max out Crysis if you are running at a higher resolution like 1080p. If you are running something like 1680x1050 or lower the 6850 might just be able to pull it off, just don't expect to be able to use high amounts of anti-aliasing. At 1080p I can barely run Crysis on the very high settings with no AA and get an acceptable framerate with a Radeon HD 6870, which is about 10% faster than the 6850.
It would help to see the full specs of the computer you are going to buy, and what screen resolution you are using. If the two cards you mentioned are your only choices, definitely go for the 6850, but if your budget allows for something higher than the 6850, you definitely should go for it. Despite being 4 years old, Crysis is still a very demanding game, most cards still have trouble with it on the maximum settings. If you are talking about Crysis 2, neither card is going to max out that game, especially with DX11 enhancements, not unless you want to run at a really low resolution like 800x600.