SLI 980s do technically provide a better frame rate on average in some games, but the stuttering and spike in frame rates means that you don't get a very enjoyable experience.
The 1080 is much better optimized for DX12 titles and features Async compute with support for Nvidia VR Works.
If you look at some benchmarks, the performance simply blows away SLI 980s in those games which do not support SLI GPUs well, this is potentially another obstacle you might want to take into account.
It is also a single card, meaning a smoother frame rate and less stuttering.
Given that you already have a 980, you COULD buy another 980, but I suggest selling it ASAP before May 27th and grabbing a 1080.
Let me know if you need any more clarification.