In my experience, systems running on AMD processors are more responsive within CAD.
If you're planning to do heavy rendering and 3D work, I would focus more on the graphics subsystem than anything else. The renderings take time to generate no matter what. I find the realtime responsiveness of the AMD setups to have a greater impact than the shorter render times on the Intel rigs.
That said, if it were my money, between those two I would go with the Core Duo (and buy double the memory with the savings - the amount of RAM you have is critical if you intend to generate complex renderings with "radiosity").
What software do you intend to use, and what are you trying to create?