The most compelling reason to go for the Titan over the GTX 680 would be for more consistent frame latencies, i.e. smoother gameplay. I don't think it's enough to justify the extra expense, since you already have one GTX 680. If you were starting fresh and deciding between the Titan and GTX 680's in SLI, then I would go for the Titan.
"When it comes to a consistent gameplay experience, the TITAN is head, shoulders and knees above its compatriots. The low standard deviation shown between frame times is a fraction of what other solutions offer, which in turn leads to significantly smoother onscreen performance. For example, the GTX 690 exhibits many of the issues normally associated with dual GPU cards like minor stuttering and outright frame hesitation, a situation the TITAN deftly avoids by virtue of being a single core solution. Gamers won’t have to wait for the latest SLI profile for optimal new game support either. So while the GTX 690 may outclass the TITAN in terms of raw framerate production, the GK110 clearly holds the edge this key area."http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/59785-nvidia-geforce-gtx-titan-6gb-performance-review-19.html