1. Go with Asus according to my personal experience. It is best than any other brand.
2. If you want from these 2 brands I will go with sapphire. It is better than xfx. In xfx many problems come with time .
I hope it helps.
I would recommend Sapphire because of its good cooling of its critical internal components like GPU, VRM and memory. I heard XFX had heat and reliability issues so i wont recommend it. Take a look at this review of Sapphire Radeon HD 7870 GHZ OC edition at : http://www.xbitlabs.com/articles/graphics/display/sapph...