580/480 8GB is a better choice especially if you want an Adaptive/FreeSync monitor. 1060 6GB has about equal performance in older titles but loses a bit in 2016+ engines/apps/games and a GSync monitor is $100+ extra compared to it's free equivalent not hampered by Nvidia tax for the proprietary board needed inside monitor for GSync to work. Equal in price, equal in performance, equal in availability I would go with the 480/580.
I bought an ASUS RX480 via amazon.co.uk (for ~AUD$265, saving ~AUD$100) back in Mar17 and it's developed a fault. (ASUS DUAL-RX480-O4G Radeon 4 GB RX 480, shipped promptly to me here in Australia) Looks like some electronic fault or some such was causing it to intermittently cease sending any video out signal, particularly when attempting to load & run 3D-graphical games. Then more recently simply attempting to play videos. A few days ago it failed (monitor signals stopped abruptly) and now it won't send a signal at all. I removed it and re-inserted an older PCI-e video card (PC running Win 7 Enterprise 64-Bit) which boots and sends the monitor video signal fine. ASUS Service Australia has advised that no global warranty applies to ASUS VGA cards, & since it was not bought in Aust. no warranty applies, which is disappointing. I expected most mainstream global brand electronic products would have a 12-month warranty pretty much regardless.