The only real difference between a mobile Core Duo and mobile C2D is that Core Duo doesn't support 64bit extensions.
That's it. So if 64bit isn't important to you, I'd go w/ the $815 option.
And as far as a 30% performance difference that dude in the Yahoo article mentioned, I think it's B.S. AFAIK there's no major architectural differences between Core Duo and C2D with the exception of 64 bit processing. Read the CNet article... they have benchmarks showing there's a less than 11% performance gap on some apps and none at all on others.