Basically, its like getting 2 car engines and bolting them together, although your top speed may not be any quicker, your acceleration will be, and the reason that a Dual Core may not be as quick as a single of the same PR is that it has more circuitry and other "stuff" to enable the Dual Core thang! Like the weight of the 2 engines in a car would also make its top speed slower although its Torque is doubled, not its HP.
Basically, my advice is, if you want just high performance for 1 application, for instance playing a game, thats it, then its gotta be a Single Core of a high PR. Although if you MSN, Vent, Firefox, FEAR, and BitTorrent all at the same time, then most definatly a Dual Core is better.
Ruan