From what I remember, the new 7xxx series from AMD has inferior image quality, compared to the 6xxx series (controversy over framerates). This might have been resolved in a driver update, but most likely the 680 has better picture quality.
There was initially a report of degraded image quality in some games. Tom's did a test on it and you could barely tell the difference. AMD tracked it down to a botched shader optimization and fixed it in Catalyst 12.3. Since then I've not heard anything