Those are very expensive. Get a GTX 670 or HD 7970 or even HD 7970 GHz. New cards will be out soon so if you want to wait, now is a good time. But you can buy any of those three that I suggested and with the rest of the money that you saved from not buying the GTX Titan or GTX 690, you can buy a replacement in a few years. But if you buy a GTX Titan or GTX 690, you'll have to spend more money to either SLI or replace it.
But out of the two that you mentioned, I would go with the GTX 690 if you do not plan to do SLI GTX Titan. SLI GTX Titan is more powerful than both a single GTX 690 and SLI GTX 690.