Efficient? Do you mean productive?
For efficient, a single core would be more efficient than either dually or quad, since the gain per core is never linier.
If you were looking at production output , that would depend what program you were using, and how well it/they were optimized for multiple cores.
You have to check to see if your programs use multiple cores or not. Some do and some do not, for instance only a few features of autocad use more than one core, most of it does not. I think the rendering portion of autocad is the only part that uses more than one core. Note that other software might make better use of more than one core. If you read the various tests that toms has done you can see which of their test software works better with a quad core and which does not.
I run autocad on a dual core computer, knowing that what I do on it does not use more than one core. When I built my machine I knew that I could more easily get a higher clock speed with a dual core chip than even a more expensive quad core chip so that is what I did. I have tested this computer running with dual cores and also with telling autocad to only use one core and I could not tell any difference.
At the current time you can save a little money by buying a dual core chip, but the i5-750 is so cheap for the performance that it offers that it doesn't make much sense to buy a cheaper cpu for a cad workstation. The 3 cad workstations I've built since I built mine have all used the i5-750 chip.