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-RX570-O4G-GAMING http://www.asus.com/media/global/products/Ig6XNsgpTunbQUJI/P_setting_xxx_0_90_end_230.png
DUAL-RX580-O4G http://www.asus.com/media/global/products/yYtddvqwPGDNde09/P_setting_xxx_0_90_end_230.png

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.