5850 vs 6850 http://www.anandtech.com/bench/Product/295?vs=291
http://www.anandtech.com/bench/Product/512?vs=539
5850 is faster but 6850 is more power efficient. 6850 used to be the most powerful card that requires only onee pcie power connector before 7850 was introduced this month.
i'd pick 6850 as the better of the two, based on power consumption.