The classic argument is get the biggest SINGLE processor system capabable of running your work
Dual processors are a solution used where the power of one just isn't enough. There will always be a performance loss as a result of management overhead of scheduling more than one CPU, however look to the price / performance curve and relate that to your budgetary constraints: no constraints - no worries
Now if you want to know which is better for your requirement, one would suggest that Opteron may be better suited. Read more at
http://www.amd.com/us-en/Processors/...6_8832,00.html