If you were to do a side by side comparison of Sapphire and Gigabyte based on user reviews of any given graphics card, more often than not Sapphire would win. Not based on anything other than their experiences. More Sapphire users are happier with their purchase than Gigabyte.
That being said, I have not had any issues with Gigabyte and have nothing against their products. My next video card purchase will probably be from Gigabyte. What you need to look at are the ports and the clocks for those two cards as those are the two main differentiators.