I could be wrong, but I'm pretty sure that Sapphire is an authorized ATI graphics card manufacturer. So there is no difference really. They use the same chipset and criteria for building the cards.
It's the same with nvidia. You can get the same model card from several different manufacturers. Abit, PNY, Asus, the list goes on and on. ATI seems to have fewer ODM/AiB relationships than nvidia.
Just remember that even though the main chip is the same, you could be buying substandard equipment when you get a cheaper card. This may not be the case with Sapphire. They seem to be a fairly large name in the ATI gpu business. It's just a general warning.