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.