Both setups should perform similarly, but take into account that the GTX 680 will run cooler, quieter, and consume less power. You also have the option of running SLI in the future.
Do you already own the GTX 560 SLI setup? If so, I would keep the GTX 560 SLI for now. I assume most AAA titles or popular games would run great with your current setup (more importantly, the games you currently own). I would wait until the next round of cards arrive, or invest the money into upgrading the CPU (if needed), getting an SSD, and much more.