SLI has strict requirements, so I would ensure you meet those before going that route.
From the purely theoretical side, SLI 970's will win in a competition against a GTX 1060, however that is only in titles where SLI is available. Since not all titles support SLI, only you can decide if you are going to play titles that will benefit from the SLI setup.
From a subjective point of view, some folks claim they don't like the performance or behavior of SLI because it doesn't feel as smooth as a single card does. Again, only you can decide if this is an issue for you.