http://www.techpowerup.com/reviews/AMD/Catalyst_12.11_Performance/23.htmlBy a lot or a little? The 680 won by slim margins (but like I said, it won), but its trump card was its power efficiency and by association, quietness. Both can be handled via proper cooling engineering of course, but public perception even in enthusiast circles goes a long way.
AMD/nVidia don't release sales numbers, do they? I wonder who has actually sold more GPUs in each performance segment this time around. The products are closely matched, so which a person buys seems largely tied to personal priorities and brand loyalty.
That said, the 680 is still $100 more than the 7970. Its power savings wont make up for that $100 difference. 7970 is a no brainer in this context.