http://it.anandtech.com/IT/showdoc.aspx?i=3769
Looks like a 2 CPU, 12 Core Westmere server is about as fast as a 4 CPU, 24 Core Opteron Server. including in virtualisation.
The Dual CPU server is cheaper and uses less power. You can use all of the technical explanations in the world about how AMD's processors will be faster, but the benchmarks do not back that up.
AMD can only compete on price against intel now. At everything else, they are worse.