A single card will draw less power than an SLI setup. On the other hand, when you can afford cards like the Titan, I guess power consumption doesn't matter at all.
Last week, someone playfully uttered the phrase “Titan junior.”
In the grand scheme of things, NVIDIA’s GTX 780 is the inevitable numbered successor to the GTX 680, packing about 34% more performance, 50% more CUDA cores, and 50% more memory. But it uses the same cooler found in the GTX Titan, as well as the same Kepler architecture. In reality, its unwritten tagline could be “The Titan for the rest of us.”
Especially since at $649, the GTX 780 costs roughly $350 less than the mighty Titan. High-end PC gaming (which we’ll define as at least 1440p, not 1080p) just got significantly less expensive.
The consensus of opinion on the web is that if you have a multi monitor setup the Titan is better but if you have a single monitor the 690 is better. Out of the two the 690 is the more powerful overall thats why the Titan is semi-officially regarded as the GTX 685.