Do you already have a 680 currently?
Using the logic that a 680 = 770, SLI 770s are a tadbit faster than the 780 Ti. But the 770 is a hair faster than the 680, so SLI 680s would likely be at about the same speed, or slightly higher than the 780 Ti.
But you also have to account for heat and power consumption, and whether or not the rest of your build supports it.
All that said, I would go with the 780 Ti.