It is the 3GB VRAM. 2GB is barely enough, I have seen my usage btw 1.5 to 2GB VRAM even for games which are not intensive at all.
Since 680 and 7970 are so close, all that a game needs to do is require 2.2-2.4GB VRAM and the 7970 will be a good 10-20% faster because of that unless you lower the settings on the 680.
It has already started to happening with 680 SLI at super high settings when VRAM limit is reached, performance hit is 15-40% based on the situation even with 680 SLI when the rez is insane.
When new games come up, the difference is likely to grow.
And although the chance may not be 100%, but it is definitely 90%+.
Just look at what happened to 4870 512 vs 4870 1gb,
460 768 vs 460 1gb, 5870 1gb vs 5870 2gb,
all these are recent cards, 1-2 years old, everybody said the same thing every time, they repeated the same mistake in the same situation and expected different results. Won't happen.
This is the same mistake and it is being repeated again.
In a time when even mid range cards have 2GB VRAM, and enthusiast cards are trying to target 3-4GB VRAM, by latest H2 2013, or H1 2014, there will be at least a few games where a min of 3GB will be required to max at 1080p 4x AA or 1440p no AA. The 7970 will be okayish then, but the 680 will be a lot slower.
Many of us would have upgraded their PCs, those who do every year. But those who upgrade once in 2 years will enjoy making the right decision then.