Yeah, erm...
Neither. The GTX 780ti is faster than a titan. The 7990 has SERIOUS heat and noise issues, AND horrible drivers that have half a dozen different problems.
If you're considering spending $1000 on a graphics card, why in the WORLD aren't you getting a 690, which is so powerful that it has its
OWN tier shared only with the 7990... which as mentioned, has more issues than a serial killer convicted on insanity.
...or better yet, wait about two months and buy a GTX 790 when they're released.