The performance gain you will get from the GTX 780 Ti is about 20%.
At 1920x1080 resolution, the GTX 780 is more than fast enough and the extra cost is not worth it.
At 2560x1440 resolution, you will get some benefit from the GTX 780 Ti card.
Two GTX 770 cards in SLI is considerably faster than the GTX 780 Ti if you are running 2560x1440 or higher. I would recommend the 4 GB models for SLI.