The R9 280X would be my choice because it's way cheaper and both cards perform quite similarly. Also you get mantle with AMD which gives AMD cards a huge advantage over Nvidia in select games.
If you have the money purchase the GTX 770 4GB version. The extra VRAM will come in handy. Many games are optimized for Nvidia graphics cards.
Recent Ubisoft games have been exclusively optimized for Nvidia hardware including the Assassin's Creed series and Watch Dogs. The 770 has recently dropped in price. Take a look:
The 770 is generally a faster card and allows you to enable exclusive settings: PhysX, Adaptive VSync, TXAA, FXAA, HBAO+, etc. You'll also have generally a quieter and cooler card, better drivers, and better dual card support.