RX580 8GB or GTX1070 8GB
From what I understand and have currently read about future AMD card releases, ETH ASIC and ETH POS. I'm thinking of selling the RX580's first. This would be on the basis that the 1070 has more mining potential and possibly a better re-sale in 6 months.
I agreed with you on keeping GTX1070 8Gb for sure. Not only about the thread of ETH asic and PoS to sell RX580 which I'm using a lot, 1070 is very flexible and could achieve high performance for most of algorithms. Before I even used 1070 to mine ETH, then I used it to switch mining between several algorithms which results to quite good profit. For the re-sale in 6 months, I dont think the price of 1070 will be reduced much. I'm also thinking about my RX580: keep them to mine (ETH+XMR) or re-sale to buy some asic miners.