Adrian's correct in that it's helpful to know which one you have, however, generally speaking the differences aren't that big.
Essentially all that tends to be different between different brands of the same graphics card (generally speaking) is what type of cooler they have (single fan, double fan, triple fan etc), as well as the factory overclock on the card.
What I mean by that: different companies will overclock their graphics cards to different amounts, and that can be different depending on which company you buy from. HOWEVER: if you feel like doing it yourself, you could simply overclock the card on your own by using MSI Afterburner (much simpler than it sounds), thus eliminating that as a potential source of value for a card.
But yeah, it would be good to know specifically which cards you're talking about.