Definitely the 6GB version. Unlike most GPUs, the GTX 1060 has a different number of shader cores between the two memory configurations. The 3GB would do fine for many titles, but in demanding titles, The Witcher 3 for example, you will likely see drops down into the mid 40's with the 3GB model. The 6GB model generally pushes around 60 in the same scenario, with drops to the mid 50's.