Brand variances have more to do with warranty and features than anything else. The warranties vary a little bit (time of warranty mainly). Also, some brands give you more stuff 'in the box' such as SLI bridges, cables, etc.
Some brand specific models will have slightly altered clock speeds, or aftermarket cooling solutions that may let one model run a little cooler or quieter than others.
As far as performance, I've never seen that there's a performance difference worth sticking with any particular brand. I've owned graphics cards from several manufacturers over the years and have never had a problem. Lately I've been buying nVidia based GPUs from MSI. They tend to be less expensive, and in my experience perform just as well (and I've...