I understand 1 has double the vram but the asus has a superior clock speed:
http://www.dabs.com/products/asus-amd-radeon-r9-270x-1120mhz-2gb-gddr5-pci-e-3-0-hdmi-directcu-ii-top-oc-91BG.html?src=1
http://www.dabs.com/products/sapphire-technology-amd-radeon-r9-270x-1020mhz-4gb-gddr5-pci-e-3-0-hdmi-oc-90P0.html?refs=55570000&src=3&awc=3044_1385583635_5934a3001b0597cd3f3565f5eee15ee1&referrerid=AW&utm_source=awin&utm_medium=affiliates&utm_content=AW00
cheers
http://www.dabs.com/products/asus-amd-radeon-r9-270x-1120mhz-2gb-gddr5-pci-e-3-0-hdmi-directcu-ii-top-oc-91BG.html?src=1
http://www.dabs.com/products/sapphire-technology-amd-radeon-r9-270x-1020mhz-4gb-gddr5-pci-e-3-0-hdmi-oc-90P0.html?refs=55570000&src=3&awc=3044_1385583635_5934a3001b0597cd3f3565f5eee15ee1&referrerid=AW&utm_source=awin&utm_medium=affiliates&utm_content=AW00
cheers