It's always best to wait until the end of the product cycle, rather than be one of the first to buy the new generation. The end is when the best, most perfected GPUs come out, after they've discovered and fixed the flaws. You can see this in the Fermi generation (GTX 480 vs. GTX 580) and the Kepler generation (GTX 680 vs. GTX 780). In all of these cases, it was clearly better to wait it out.
So my recommendation is to go for the best the current generation has to offer, the GTX 780 Ti, and wait a couple years for the Maxwell refresh, the 20nm GTX 980s.