For Watch Dogs, the GTX 770 would be faster; especially the 4GB version.
If you're getting the 2GB version though, then the frame rate might be choppier due to the 2GB frame buffer, and you can forget about using ultra level textures as that requires a 3GB frame buffer.
In fact, I would recommend getting the 280x over the 2GB GTX 770, as the performance of the Radeon will increase as AMD releases more optimized drivers and Ubisoft patches the game.. Plus with 3GB of VRAM, the 280x can use ultra level textures which makes the game look significantly better.
But if you can afford the GTX 770 4GB, then that would be the best..