Get Sapphire Radeon HD 7850 2GB GDDR5 OC Edition
Difference Sapphire Radeon HD 7850 2GB GDDR5 OC Edition will perform little better due to the factory overclock
If you OC and the price of the OC version is not too much more, get the OC version. If you don't OC and the price diff is larger, get the normal one. The OC one has a better cooler. So it depends on what you are willing to pay for.