They are no different from on another, Here is how you choose, what one will look better in your rig, thats how you choose, because these cards are so alike it doesn't really matter what one you get.
If one of the cards has unlocked voltage, you should choose that one.
Otherwise, read reviews on NewEgg and compare the specs and warranty. Factory OC's can be nice but should not be the determining factor. Check the reviews for overall OC'ing and temps.