I'd prefer the 6GB 1060 for longevity. 3GB of VRAM is pretty much the bare minimum for 1080p gaming right now unless you are willing to drop things like texture settings to low or medium, and I could see the same thing happening to 3GB cards in the near future. 6GB gives you more headroom for higher resolutions and for higher resolution textures. The 6GB version of the 1060 is also about 10% faster than the 3GB version due to having more CUDA cores so that's another plus.