id say definitely the hd6870 if you have an appropriate PSU, that sapphire cooler works well. The only downside to that card is the lack of included crossfire cable, but those are frankly a dime a dozen on ebay.
Not listed for the 6870. Core is 900 (higher), but memory is Lower @ 1050
Features not listed for the 6870. The 6850 may have better features and allow better Overclocking.
Might want to google reviews to see how the really perform in games. If not much diff I'd also go with the 6870.