dont really knw wat you'r askin for...
to put it simply, if you are buying a complete new machine then go for i3, if its just the processor then of course core 2 duo would be a logical choice.
the difference?
i3
-32nm
-integrated graphics
-PCI-e 2.0 controller, couple of em come with 2X8 along with da 1X16 support
-2channel integrated memory controller
-hyper threading
core 2 duo
-65,45nm (needs more power n generates more heat)
-of course no concept of integrated graphics
-no PCI-E controller
-no memory controller
-no HT
there's a lot more.. but i think this tells enough to decide whether you should go for an i3 or core 2 duo... i3 is a much more refined and mature architecture and i love what Intel is offering.. if only they could do something about the naming scheme :S