Seems to me they are all functionally identical EXCEPT:
some manufacturers may include a slight overclock;
some manufacturers include a game or two (or something else)
some manufacturers have diff. warranties.
Essentially, every XTX has the same core chip(s) on it. so ultimately they are the same thing unless the manufacturer does something listed above. When shopping around, keep that in mind. I'd suggest going with a combination of price, warranty, and included items because aside from that, it's the same card no matter who you go with. As for the overclocks, you can do that yourself just as well... so looking for a factory OC card wouldn't be an issue for me.
I went with the ATI brand because I found one on ebay, new in box, for only $430 and the thing is just AWESOME....