Single Quadcore or Dual Dualcore?

Physically speaking, the latency between two separate processors separated by centimeters is a knock on multi-CPU configurations in itself. However, these configurations typically have independent cache, which is different than single-die / dual-die multicore system. These shared-cache architectures gain considerable performance increases when the right application is running (e.g. one with few-to-no cache misses, using all the cores), but the cores are still vying for a shared resource which can cause the opposite to happen - terrible cache misses that kill performance.

Unfortunately every board and architecture is different. Your question can be answered or at least guessed at for a given application. As it stands, though, the jury is out when it comes to raw performance.

Cost, however, is another matter. I think it's obvious given the proliferation of multi-core systems that cost is in the quad-core's favour.

One major benefit to a single quad core would be power and heat/cooling savings overall.