The 6850 is better but it is NOT cheaper. I just bought a 7770 instead of a 6850 because I really didn't even have the $105 that I got my GPU for. A 6850 is worth the extra cash though, if you have it.
Suppose I'll mention that yes the 7770 does indeed use less power meaning less heat than a 6850 would generate. I have MSI's most generic 7770 and it runs at a cool 50C playing any game on any settings. A more expensive 7770 would probably run cooler, but you don't really need it.