I’ve never heard of Virtual Iron.
VMware is expensive, but has many features which make it the more desired enterprise class virtualization platform.
With my limited testing of Microsoft Hyper-V it seems stable. From reading the Virtualization chat questions, it seems as though Microsoft is pushing the systems management piece which is a good idea, but they are still new to virtualization and so they do not have the advanced features that VMware can offer.
I’ve also never heard of Virtual Iron. VMware ESX is the big player in the Vritualization market these days. They have a more mature project then Hyper-V.
VMware has vMotion which allows you to move a guest OS from one host to another without any down time. You also have Storage vMotion which allows you to move a guest machine from one disk to another without any down time.
We have just begun migrating into a new VMware ESX environment.