I am not very knowledgeable in this field, but I did a quick search on google, and I found:
http://www.codinghorror.com/blog/2007/09/choosing-dual-or-quad-core.html
Apparently, clock speed would give the greater advantage, but if the clock speed were the same, then the more cores you have would increase your performance.
This means, clock speed > # of cores
The benefit of choosing a quad core over a more powerful dual core may be the following:
For 3D/compositing workstations, a quad-core CPU (or dual-CPU quad-core) does substantially speed up rendering.
So it seems that for general usage, a higher clocked dual core would be more efficient than a lower clocked quad core.
Hope that answers your question.