Searching around I couldn't quite come up with an answer.
I'm building a new system and I ran into this problem and I can't quite figure it out. How do they perform? Is the price difference worth the performance difference? Are they even compatible? Why are they named the same? http://ark.intel.com/Compare.aspx?ids=27251,42809
I was going to go with the Pentium Dual Core for now to keep costs lower and upgrade it in a couple years with something else. But should I go with the C2D instead? The price difference is huge.
The Core 2 Duo E6700 is an old CPU. That was the high-end mainstream CPU in 2006. It's got a lower clock speed, lower bus speed, and overall is just slower.
The Pentium Dual-Core E6700 is a newer CPU, it was released last year (2010). Higher clock speed and FSB. The Core 2 Duo is only more expensive because it's hard to find, and it's an in-place upgrade for people with 2006 computers who don't want to change out motherboards.
For a new build, get the Pentium Dual-Core.