Core 2 Duo is 64 bit, Core Duo isn't. Leopard is 64 bit. It will be more future proof in that regard.
(there have been 3 generations of core 2 duo machines now, you should be able to get a second hand core 2 duo as a compromise)
The latest core 2 duo can take 4 gig of ram, the previous core 2 duo could only take 2 gig.
The latest core 2 duo has a much better graphics card (x3100), all the other macbooks have the GMA950.