First of all, hello... and excuse my poor english.
980 Ti performs like 2 x 780 Ti in SLI, but without the SLI troubles, double vram and DirectX 12 full support.
Even that DX12 will gather the whole vram memory, If you go for another 780 Ti, you will probably need to change your PSU.
Many games demand more than 6 gb of vram playing on high res... and Directx 12 allows more than 63 million of polygons per second (over 6-12 times more than Directx 11), so that means that future games could use more vram because of the level of detail, just like Skyrim with mods (4-5 gb of vram used).
So, if you want to play games on high res and a gpu for a long time, sell yours and go for the 980 Ti... or wait till AMD releases the 390.