If you are looking at the reference models then they are pretty much the same.
The difference will be in 1. the warranty offered by the specific manufacturer 2. The accessories included ei. cables/adapters 3. The quality of parts used in the card
Most people will have the "brand" they stick with or the ones they trust, mostly based on personal experience. I have heard good things about Sapphire, XFX ("mainly because in the past they have had lifetime warranties") , MSI
I can vouch personally for the Sapphire HD 7970, this is the one I purchased and have been using for a couple of weeks and have had no issues at all, runs perfect nice and cool - Temps playing Star wars: tor at 50% Fan hover right around 55-60C - with my old gtx 480 at 80% fan speed it would run 75c >.<