Comparing a GTX780Ti to GTX760 SLI doesn't make a lot of sense - approx $670-700 vs $500.
Comparing a GTX780Ti to GTX770 SLI however, makes a lot more sense - approx $670-700 vs $660
The GTX780Ti sits approximately between both of those SLI setups, though this will vary significantly from game to game and display resolutions. In outright FPS, GTX770 SLI is likely to be better value than a GTX780Ti, though I'm not sure the relatively small performance gains outweight the negatives - power usage, heat, scaling issues, SLI profile issues.
If it was me, I'd get the GTX780Ti, though only if I was playing at a high enough resolution to warrant it. At 1080P, it's massive overkill, and most people are still using this resolution.