Well the basic difference is that i3 are the newer models, all of them are dual core.
the Core 2 duo are older like 2-3 years depends which you choose.
i3 have 2 versions last years that are marked with 3xxM and this years 2xxxM(most often 2310M)
A new i3(2xxxM) is better than any i3 from last year and much better than core 2 duo
Here is a rule of thumb:
i3-2xxxM @ 2.100 Mhz is more than i3-3xxM @ 2.400Mhz is more than Core 2 duo @ 2.800 Mhz.
Also give the entire chart with considered models and why you need it. Because computers are more than just processor.
Depends on the budget and the scarce data i give you this recomendation:
Get i3/i5 2xxxM
6-8GB Ram
500 HDD(or more)
GPU*ATI 6470(or higher, 6570 is better,5650 is better,4870...