In most titles the two cards are within a frame of the other, I would say due to the 1060 6GB being ahead slightly (very slightly) it is the optimal choice for those who play a larger library of games, however you should base it on the games your playing or plan on playing and which card is better optimized for those titles. Refer to this list and various others to make your choice.
https://www.techspot.com/review/1393-radeon-rx-580-vs-geforce-gtx-1060/
Also note that even though the market for it is dwindling, the RX 480 and 580 are still slightly above their actual value for gaming. Nvidia has been affected by this as well but not to the same degree.