Speed and memory
In our performance index the GeForce GTX 980 scores 11 and the GeForce GTX 1060 6GB scores 9.5, which makes the GeForce GTX 980 about 16% faster. That is a visible but modest gap, usually 5 to 10 fps in a game that runs at 60.
The GeForce GTX 980 has 4 GB of video memory and the GeForce GTX 1060 6GB has 6 GB. That difference matters at 1440p and in new games at High textures, and it can outweigh a small speed advantage as the card ages.
In our estimates for popular games at 1080p on High, the biggest gap is in Call of Duty (21 against 26 fps), and the smallest is in Red Dead Redemption 2 (40 against 36 fps). Games that are light enough to run above 200 fps on both cards show little difference at all.