Yeah I would go with the GTX 970. An extra .5GB of regular speed VRAM and .5GB of the slower speed seems like the better deal. Not a fan of the GTX 970s VRAM setup but even my GTX 980s with a full 4GB@ full speed has issues with some games maxing out said VRAM. A GTX 1060 3GB is going to be even worse off in those situations. If you could afford a GTX 1060 6GB or shave some off your build price somewhere else, the 6GB variant would be the best choice of all.