You can reference to his thread:
http://www.tomshardware.com/answers/id-3148057/gtx-780-gtx-1060.html
it has some answers too
Basically some people debate whether the 3GB vram is enough for the games in today's AAA game titles, which I think it is just enough in 2017, but might not hold itself in 2018 or later. Two 780s in SLI could yield you good performance, but would require a beefy PSU, as well as a motherboard that supports SLI.
Some people rather go with the 1060, as it's less complicated. Others would rather the 780 SLI as they favor the better performance.
However, it also does matter which game you play. Some games aren't optimized for SLI, and give you very little additional performance over a single card. Some games are optimized for SLI, and do give you anywhere from a 150% to 180% performance boost. Maybe in some cases a little more than that, but never 200% or more.
You'll just have to decide if it's worth it yourself. 2-way 780s do require a good psu as it is a 250W card, and 2 of them would draw 500W at load. You would require ar least an 800W or more psu to be comfy, as well as get an SLI supported board.
However, the single card 1060 does have a much lower TDP, a quarter of what you would have with two 780s. (120W) And this allows you to get a 450-500W or more psu. Plus, it overclocks better and it doesn't require an SLI supported motherboard.