If you already own a 690 which supports G-Sync, I think other than power consumption you wouldn't see drastically higher performance on the current 780ti.
If your 690 caps out on the vram and becomes a problem, the 780ti with the larger amount of vram would be the answer. If you take the route of replacing the GPU due to insufficient vram, I would wait for the 6gb version of the 780ti, instead of buying the 3gb version, which should be released soon.
That would give you a lot more leeway for future games which will most likely increase the vram requirements as more and more processes get moved to the GPU along with higher quality textures as well as affording the opportunity for further SLI configurations.