The GTX Titan has a 26 MHz higher core clock speed than the GTX 780, but the GTX 780 has 144 more Texture Mapping Units than the GTX Titan. As a result, the GTX 780 exhibits a 118 GTexel/s better Texture Fill Rate than the GTX Titan. This still holds weight but shader performance is generally more relevant, particularly since both of these GPUs support at least DirectX 10.
The GTX Titan has a 26 MHz higher core clock speed than the GTX 780, but the GTX 780 has 48 more Render Output Units than the GTX Titan. As a result, the GTX 780 exhibits a 40.1 GPixel/s better Pixel Fill Rate than the GTX Titan. However, both GPUs support DirectX 9 or above, and pixeling performance is only really relevant when comparing older cards.
The GTX Titan was released less than a year after the GTX 780, and so they are likely to have similar driver support for optimizing performance when running the latest games.
Both GPUs exhibit very powerful performance, so it probably isn't worth upgrading from one to the other, as both are capable of running even the most demanding games at the highest settings.
The GeForce GTX Titan Black Edition and the GeForce GTX 780 SLI have the same amount of video memory, but are likely to provide slightly different experiences when displaying game textures at high resolutions.
The GTX 780 has 240.8 GB/sec greater memory bandwidth than the GTX Titan, which means that the memory performance of the GTX 780 is massively better than the GTX Titan.
The GeForce GTX Titan Black Edition has 2880 Shader Processing Units and the GeForce GTX 780 SLI has 4608. However, the actual shader performance of the GTX Titan is 2822 and the actual shader performance of the GTX 780 is 4156. The GTX 780 having 1334 better shader performance and an altogether better performance when taking into account other relevant data means that the GTX 780 delivers a massively smoother and more efficient experience when processing graphical data than the GTX Titan.
The GeForce GTX Titan Black Edition requires 250 Watts to run and the GeForce GTX 780 SLI requires 500 Watts. We would recommend a PSU with at least 600 Watts for the GTX Titan and a PSU with at least 750 Watts for the GTX 780. The GTX 780 requires 250 Watts more than the GTX Titan to run. The difference is significant enough that the GTX 780 may have an adverse affect on your yearly electricity bills in comparison to the GTX Titan.