Go for the 6 RX 580s since they are for the best for mining ETH and other altcoins. However you might have a hard time finding it since it is either short on stock or it has a higher price than usual depending on the country. As I have checked on newegg an RX 580 8gb costs around $300 USD which is mcuh cheaper than a few months ago but still higher than MSRP. I would stay away from the GTX 1060 specially the 3gb models if it has the SK Hynix memory as it is not utilized for mining and you will have lower hash rate from it. If you will get a GTX 1060 that does not have an SK Hynix memory then go for it.
Do the 6GB 1060s have the Hynix memory?
From what I have heard most of the GTX 1060 3gb has the SK Hynix memory. There are some 6gb variants that has that mostly the GTX 1060s form Palit, Galax and Zotac. And since in the region that I am in these brand has the most stocks. However there is really no telling if the card has SK Hynix memory if you don't try the card first before buying it an most of the retailers does not do that and if you try the card most likely you would need to buy that from them. That is why I would recommend to go for the RX 570/580 over the GTX 1060 or if you have the money get the GTX 1070 because they use Samsung or Micron memory.
Thanks, I think i’ll get 6 x 580s just now, having a look to see what the prices are and if I can buy 6 together.
Don’t want to be stuck with a 1060 with Hynix ram for sure