Easy choice. The Titan black is an overpriced under performer for that price toy for rich kids. Pass on it. Save yourself some money. Price/Performance doesn't add up just like the first Titan. Get a 780ti if you want to spend that kind of cash.
760GB 4GB vs 780 6GB are you real choices. This is easy again.
The 760s in SLI will get you better performance but not by a large margin. You will also have more heat, power usage, risk of driver problems and micro stutter. Not all games or apps can support SLI.So you might not be able to even use it 100% of the time.
The 780 6GB version will give you more Vram because 4GB+4GB=4GB in this world, Less power, less heat, simple driver installs, 100% app and game capability, future room to upgrade to SLI if you only get one now and will cost about the same as the 760s in SLI.
And if you decide to go full water cooling you only need one block instead of 2.
THe 780 6GB is the clear victor in this round.