It depends a lot on what you are doing with the PC. Your comparing a single core to a dual core, and even though pentium-d's are the worst of the dual cores, if you are running SMP aware programs, such as video encoding, the pentium-d will still be better despite being a flamethrower. If you are gaming on the other hand, the opty will blow it away.
Pentium-D advantages-dual core
Opty advantages- faster for non-smp aware applications, runs much cooler, uses less power, more overclockable.