In terms of performance the HD 6870 would be better of course.
As for which brand is better, that depends on the warranty and any particular bundles.
AMD video cards are actually manufactured by only a few OEMs (Original Equipment Manufacturers). The OEM manufactures the card, then slap on the appropriate brand name sticker for their client. As an example, PC Partners (unless they changed their name) is the parent company of Sapphire, but they also manufacture cards for other brands. Another example is Pine Tech, the parent company of XFX.