RX 480 hands down if I were on the market for either card, although I'm not as I am still happy with my R9 290 Windforce card, which only seems to be getting faster and faster with DX12/Vulkan.
The AMD architecture is simply more powerful in DX12/Vulkan, and that's the immediate future. I believe nVidia was simply not expecting DX12/Vulkan when the Pascal chip was on the drawing board, and they've been caught with their pants down. Their GPUs seem to be more energy efficient, but that's because they're missing crucial components, mainly a hardware scheduler and asynchronous compute engines.
Finally, AMD now owns >75% of the overall gaming market (Consoles + PC), and that figure is only set to increase to >90% within the next two years. Game engines are already being optimized for Radeon architecture in the current and next generation consoles first, and nVidia is an irrelevancy to the developers. nVidia is only going to be scrambling more and more to fix games that are broken on their GPUs. My thoughts are greatly explained/expanded up on in this video: