While $300 is indeed a good price for 480/580 8gb cards, I see no point buying them for $400. That's a GTX 1070 price, and I think it's just a better card overall. Better in gaming, and better in most mining algos. I mostly have 1060s and 1080s in my nvidia rigs, but just yesterday added a 1070 to one of them. Without much tinkering, it's mining ETH at 31 MH/s (+700 MHz Micron memory) @ ~110W, so pretty much on par with Polaris. It's a lot faster on Equihash though (~450 sol vs 300 sol). Slower in cryptonight (no one is mining it with GPUs now, but still it's a mainstream algo so should be mentioned). And while there are of course niche algorithms that run faster on amd cards, for the past year or so nvidia did have more of those. Lbry, Lyra, Skein, Neoscrypt - those are just what came to mind instantly. I can't recall anything like those for AMDs at the moment (they most likely exist, I just don't know or don't remember any).

I'm not a fan of anything and don't care about brands at all, but when presented with a choice between amd 580 or nvidia 1070 for the same price — I'd definitely get the nvidia. Simply because it's a more powerful gpu overall.