Do you plan to run more than one virtual machine at a time? If not, it's likely you'll be better of with the Dual-core, higher clocked CPU.
For a very pertinent comparison see the article Comparing Intel Dual Core and Quad Core using VMark
Edit: as DMA pointed out, what is best will depend on your usage requirements. If you tend to run several processor intensive things concurrently (or an intensive, multithreaded process), the extra cores may be better. When considering 'concurrent' this includes processes running inside the VM and outside.
If most of the time the CPu is getting hit by just gcc inside your VM, and whatever else you happen to be running in your host OS, I'd say a dual core would probably suffice - especially if there are cost considerations.
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Your original question was off-topic per the SuperUser FAQ, which explicitly states that SuperUser is not for buying recommendations. However, I think your question is a good one and am curious myself, so rather than voting to close it, I edited it to be more general about the performance differences between a quad and dual core processor in virtualization, rather than just an individual buying recommendation. This should keep it from being closed as off-topic, while still getting you the information you need. :)
– nhinkle – 2010-09-15T04:54:56.087