if 6GB version have faster memory than probably, else it will mine the same.
Even if the memory is the same, the 6Gb has a different clock and it should perform at least a little bit better. I can make you an example and compare this 3 Gb and 6 Gb of 1060 cards with the Rx 570/580 series where the 4Gb cards can make almost the same as 8Gb but the 8 Gb version can make a bit more and perform well because the memory clock standard for 4 Gb is 1750 and for 8 Gb cards is 2000. The same difference should apply to Nvidia 3Gb and 6Gb versions.