It depends if the applications you're running can take advantage of more than one core. If an application can use all four cores when running then it will run better in a quad core than in a dual core. If it can only use one or two cores it will probably run better in a dual core if it has more clock speed. But this is not linear because it depends in the architecture of the processor. For instance, the old Pentium 4 had clock frequencies up to 3.6 GHz, if I'm not mistaken, but my i5 processor using only one core at stock speed, 2.6GHz will beat the P4 everyday by far.
In the other hand if you multicast a lot, have a lot applications running at the same time, you'll be better with a quad core.
So the best thing is to check how many cores the applications you're planning running can use and check benchmarks for those apps and see which processor is the more efficient having in consideration the multicast factor.
Nevertheless if you have the money you should buy the quad core since more and more applications can take advantage of multicores.