Dual xeon usually... it's a server cpu... higher oc yeilds usually. The price is never right. I've not heard much positive hype about Intel dual cores at all. I'd suggest AMD for performance AND price reasons.
Pentium Dual Cores have to deal with many many issues. They are no match for AMD's Dual Core Athlon 64 X2s because of inherent Pentium design problems, and the Athlon 64 X2s' better architecture.
But to answer your question, A Pentium-D at the same clockspeed as a Single-core Xeon will out-perform the Xenon.
wow, dual core is better than 2 CPU? well i am getting 2 Xeon on 1 board, since there is no dual core Xeon out yet. But i am getting a workstation, i hope I didn't pay the extra money for worse stuff...