If you're starting from scratch, the 2700K will probably cost less overall.
Jim is right.
There are a few scenarios where the six cores will benefit you, but not a good value for most people.
Check out Tom's Hardware charts.
At this moment you have to rephrase your question to:
i7-2700K versus i7-3930K
The i7-3930K is similarly priced to the 970 but offers much better performance and uses a better platform, X79 with more PCIe lanes, more memory, expandability etc.
The price difference between the two is around $ 350 - 400, for the CPU and motherboard and disregarding other components. The performance difference depends on your editing needs, but will probably amount to around 20% on usual editing jobs. The major performance increase will come when you use the additional PCIe lanes and extra memory and then the performance increase may go way up. No hard figures yet however.