Even if the epoch benchmark issue with the Claymore miner is not fixed or ETH goes to POS by that time, two RX 4XX/5XX card will easily do 620 H/s on ZEC or ~30% better than a 1070, or ~10% better than a 1080. Results for most other algoriths is similar. The bottom line is I can mine more coins for the same amount spent on cards with the RX 4XX/5XX than any Nvidia 10 series cards.
why you compare TWO RX4xx/5xx with ONE 1070? you think you will never pay off the investment? In case you never can get the money back from investment it makes anyway no sense to start even mining
After the investment is payed off you make with ONE 1070 ~ 470 Sol/s and with ONE RX4xx/5xx just ~ 320 Sol/s - so after investment is payed off, with which hardware you can mine more and get better profit?
Or say it in this way, with 4 x 1070 i mine nearly the same than someone with 6 x RX4xx/5xx
I guess you missed or don't understand this part: The bottom line is I can mine more coins for the same amount spent on cards with the RX 4XX/5XX than any Nvidia 10 series cards. Even after the cards are paid off, I will still make more coins going with RX 4XX/5XX than I can buying 1070's or any other Nvidia 10 card and if the profitability drops, I will still be able to make a profit where a single 1070 won't.