For this generation; I've had the privilege of owning the 7850, 7950, 7970, and the 680. Between the 680 and 7970. I feel that the 7970 is the better buy only because it is the cheaper of the two. At reference clocks, the 680 is faster on any given single monitor resolution however once overclocked, the 7970 is generally faster considering it is voltage unlocked and can be pushed quite far from its standard clocks thus vastly improving performance exceeding that of even an overclocked 680. Keep in mind that the 680 is voltage locked therefore it is limited in how far you can overclock it. The one exception to this is the 680 Lightning from MSI. It is the only Kepler card with true voltage control and depending on sample can potentially beat a good clocking 7970.
In the review below, they pit the 680 Lightning @ 1311MHz/1747MHz against the 7970 @ 1250MHz/1670MHz.
http://www.xbitlabs.com/articles/graphics/display/msi-n680gtx-lightning_7.html#sect1
In the majority of the tests, the overclocked 680 Lightning is edged out by the overclocked 7970. I should also mention that the Nvidia driver used in the review is an older version but nonetheless at those clocks, the 7970 is faster
Keep in mind that only the Lightning 680 is capable of these clocks with the exception of maybe a few other golden samples running on stock voltage.
