The GTX 690 has a 40 MHz higher core clock speed and 80 more Render Output Units than the GTX 780 Ti. This results in the GTX 690 providing 75.1 GPixel/s better pixeling performance. However, both GPUs support DirectX 9 or above, and pixeling performance is only really relevant when comparing older cards.
The GTX 780 Ti was released over a year more recently than the GTX 690, and so the GTX 780 Ti is likely to have better driver support, meaning it will be more optimized for 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 GTX 690 has 5120 MB more video memory than the GTX 780 Ti, so is likely to be much better at displaying game textures at higher resolutions. This is supported by the fact that the GTX 690 also has superior memory performance overall.
The bottom line is Nvidia is pulling out all the stops with the GTX 780 Ti in an effort to shame the R9 290X, and once again establish itself as the king of the single-GPU space. It should be noted that the GTX 780 Ti does not offer Double Precision compute performance like the GTX Titan, so CUDA developers will still prefer that card. The GTX 780 Ti is made for gamers, not scientists. I should also point out that the GTX 780 Ti supports quad-SLI, just like the GTX Titan, and the GTX 780 does not.Regardless of which one edges out the other, both are top-end cards.