There is a reason as to why we are being given dual, quad and more cores rather than single cores with higher speeds but that does not mean they are better than single cores, if we could get the speed, I think IBM had a single core running at 6 GHz, I would go for a single core.
Although Intel says that they allow to run more applications , more games, faster processing etc, how many software developments actually support parallel processing?
In benchmarks the quad would be better so if you can afford a quad get one.But otherwise I would be happy with a dual. (or a very fast Single core)