AT already compared 12 tread Xeons agains 12 cores Opterons:
"The bottom line is: is this twelve-core Opteron a good deal? For users waiting to use it in a workstation we have our doubts. You’ll benefit from the extra cores when rendering complex scenes, but in all other scenarios (quick simple rendering, modeling) the higher clocked and higher IPC Xeon X5600 series is simply the better choice.
Applications based on transactional databases (OLTP and ERP) are also better off with new Xeon. The SAP and our own Oracle Calling Circle benchmark all point in the same direction. Intel has a tangible performance advantage in both benchmarks.
Data mining applications clearly benefit from having “real” instead of “logical” cores. For datamining, we believe the 12-core Opteron is the clear winner. It offers 20% better performance at 20% lower prices, a good deal if you ask us. Intel’s relatively high prices for its six-core are challenged. The increased competition turns this into a buyers market again.
And then there is the most important segment: the virtualization market. We estimate that the new Opteron 6174 is about 20% slower than the Xeon 5670 in virtualized servers with very high VM counts. The difference is a lot smaller in the opposite scenario: a virtualized server with a few very heavy VMs. Here the choice is less clear. At this point, we believe both server CPUs consume about the same power, so that does not help either to make up our minds. It will depend on how the OEMs price their servers. The Opteron 6100 series offers up to 24 DIMMs slots, the Xeon is “limited” to 18. In many cases this allows the server buyer to achieve higher amount of memory with lower costs. You can go for 96 GB of memory with affordable 4 GB DIMMs, while the Intel server is limited to 72 GB there. That is a small bonus for the AMD server."
The funny part is that on the PC market the choice is clearly the 6 cores Phenon for $325, on the server segment the 12 core Opteron is very competite, but on the workstation market things are not so clear, and cost, power envelop and performance must be taken into fine consideration before purchasing.
This article is not biased towards Intel: it only points convenintly towards one the few market segment that Intel has a product worth mentioning ( the other being extrem low power no compromise taken PC with the i3...