If you're running a 1920x1080 monitor at 60Hz refresh:
Assuming your monitor has GSync, get the GTX 1060 6GB
Assuming your monitor has FreeSync, get the RX 580 8GB
Assuming your monitor has neither, get whichever of the above two cards is cheaper.
If you're running a 1920x1080 monitor at 60Hz refresh:
Assuming your monitor has GSync, get the GTX 1060 6GB
Assuming your monitor has FreeSync, get the RX 580 8GB
Assuming your monitor has neither, get whichever of the above two cards is cheaper.