id have to say core 2 duo... just because its 65nm and runs cooler at a higher speed.
o well... it still runs faster per clock for some operations.
Xeon is a server type of processor, ofcourse the price is higher. I believe it is a bit better than Core 2 Duo.
Just in case anyone was wondering, a Core 2 has a larger die (more transistors), potentially faster FSB, potentially more L2 cache, 2 more stages in the pipeline, and additional simple decoder, more scheduler entries, more FP units, more SSE units, an additional ALU, and a bigger reorder buffer. As Omega said, it's basically a Core but on steriods.