Assuming you do not hit that 3.5GB gate on the GTX970, GTX1060 6GB and GTX970 are quite near in performance. GTX1060 is only about as fast a GTX980, which is not far from GTX970.
However, if you are in the market looking for a new GPU, always look for newer one.
The newer is always better even if the brute power is somehow quite similar.
You have at least always less power consumption and longer driver support (counting from the day you bought it).
Avoid GTX1060 3GB, the difference is not only on the VRAM size!