I think the W chips are designated workstation as 130W TDP is alot of heat for a 1/2U chassis.
The processor are both based on the same architecture so there won't be any differences other than the ones you already know. I'm sure both will perform well for your application.
If you factor in the turbo boost on each chip they end up performing much closer than you would expect from the stock speeds. I think the X5570 would spend most of it's time at 3.2Ghz and the W5580 at 3.33Ghz.