Oracle recently released an update to VM Server for X86. The company hopes that the improvements in its user interface; storage and backup performance; drivers for Windows -- that have been validated by the Microsoft Server Virtualization Vendor Program (SVVP); and the inclusion of a large number of application templates to improve application integration will help it win business over VMware, Citrix, Microsoft and Red Hat.
Microsoft and Red Hat make similar claims about their software stack.
This reminds me of the Windows versus UNIX or Linux wars the industry saw ten years ago.
The criteria should include ecosystem support; availability of management, storage and security tools; and support by suppliers of the operating systems, development tools and applications in the company's portfolio.
If your company heavily relies on Oracle software, this hypervisor just might be the best choice. If the IT infrastructure, on the other hand, is composed of a mixture of operating systems, application development tools and applications, each coming from a different supplier, the choice is much less clear. It would be wise in this scenario to select tools that are supported by the largest community.