Both are very old processors, but go with the Dual Xeon, it will allow you to use ECC RAM.
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If you're looking at providing VPS service and you're not super worried about CPU performance, I would suggest looking at the maximum RAM supported by the mother board. With Xen/KVM, you can't share RAM and even with OpenVZ, you're limited in the amount of active processes you can have. By having a lot of RAM, you can provide reasonably sized instances from a RAM perspective. Loads are not likely to be CPU-bound.