The quad core scores maybe 16% better in the Passmark benchmarks, but---
The i5 can be boosted from 3.2 to 3.46 (single core Turbo). That 3.46 is a 30% advantage over the 2.66 of the quad core.
I regularly see comments from quad core owners saying they wish they had stayed with a higher clocked dual core.
I'd go with the dual core unless you can find convincing evidence that your particular applications can in fact benefit significantly from 4 cores. You would then have to decide how often you use the applications that might benefit.
Here is a long thread on dual core versus quad core: http://forums.anandtech.com/showthread.php?t=232212