I have the option of getting either of these two 4GB ASUS RX 5X0 cards (both in stock, have put on "reserve") at the same price ($199.99 USD):
ROG-STRIX-RX
570-O4G-GAMING

DUAL-RX
580-O4G

Power consumption is not a concern. Will be used to mine ETC, ETH, XMR. Any reason to get one over the other? Will be using ethOS.
The Engine Clock on the 580 is 1380 MHz vs. 1310 MHz on the 570, and the 580 has 2304 Stream Processors vs. 2048 on the 570. I don't notice any other differences.
DUAL RX 580? They still doin that? I tought they change for Expedition edition...
Strix are good one, but they comes with Elpida, Samsung and sadly with Hynix too.
XMR: 830H/s with Samsung, 820H/s with Elpida both 470.
Dual is pretty bad... I have 480 8G, but I have 835H/s it cost me 40 bucks more than 470 strix... And O8G dual can't handle memory overclock with performance timings, so I return it. With uber-mix 3.2 it was only about 700H/s...
Strix has much better cooling... those fans are the best you can have. Sadly there is no heatsink on memory, but no problem so far.
Dual cooling is noisy and shitty... It could be better on the new expedition edition, but it seems to have same 80mm shitty fans.