Think about it. If a single GPU is as fast as the two in your system, and games don't take full advantage of more than one GPU, then you're better off with the single GPU. Also, since games that use SLI do so reliably up to only 2 GPUs, you are currently at your max SLI configuration. Getting a single 780 ti gives you the same performance but with room for growth in terms of SLI.
To me the 780 ti seems like a no-brainier. Even if you get another 690 for 200 less, you'll still pay 400 - 500 for little to no performance increase, and be left with no room for growth in terms of SLI.