Depend by what you needed for!
The Titan Black and the 780 Ti use the same GeForce drivers, so they will perform equally well in gaming. The Titan Black just has the added feature that allows you to enable 1/3 DP performance (left off by default, but you can change in the control panel, since it will lower the clocks a little bit, or at least lowered the clocks on the original Titan).
If you are not using over 3GB of VRAM, the only performance difference between the 780 Ti and the Titan Black will be based on clock speed differences by which:
780 Ti Kingpin > 780 Ti Classified > Titan Black HC > 780 Ti SC ACX > 780 Ti SC > Titan Black SC > Titan Black > 780 Ti (if using base clocks)
780 Ti Kingpin > Titan Black HC > 780 Ti Classified > 780 Ti SC ACX > Titan Black SC > 780 Ti SC > Titan Black > 780 Ti (if using boost clocks)
780 Ti Kingpin > 780 Ti Classified > rest of the 780 Ti's + all Titan Black's all being equal (if considering board design and one's own overclocking with one's own cooling solution)
But if you plan to game at 4K, you will likely use over 3GB of VRAM, especially in newer games like Crysis 3 and BF4, and thus will need the Titan Black to prevent losses in performance, especially when you use more than 1 card and thus have a higher potential performance. I can't really comment on 2560x1440/1600 solutions as I have not seen how much VRAM those solutions use. Here is an article on how Crysis 3 and BF4 use VRAM:
http://www.digitalstormonline.com/unlocked/video-memory-usage-at-4k-uhd-resolutions-idnum146/
Keep in mind, if you scale your resolution in BF4 beyond 100% (SSAA), you will need more VRAM. The AA used in the article is the weaker MSAA featured in the AA Deferred settings. People have reported running out of 6GB of VRAM and BF4 crashing with 2560x1600 resolution and 200% Resolution Scaling (4x SSAA), so you can still definately max out your VRAM and still not experience all games have to offer.
https://battlelog.battlef...2955065218408734284/3/
My advice:
1080p and below, stick with 780 Ti.
1440p/1600p for current games, stick with 780 Ti.
1440p/1600p for future games, consider a Titan Black.
2160p for current high-end and future games, get a Titan Black.