This is a great article on why one Microsoft partner, MaximumASP, went with a Hyper-V based solution for their new scalabe virtual hosting service.  While MaximumASP is Microsoft partner, they evaluated almost every vendor in the segement before deciding on the Microsoft solution.  The insights on hosting virtual machines and the benefits of integrated physical and virtual management are invaluable.

 http://rcpmag.com/Articles/2009/06/01/Finding-a-Virtual-Solution-in-Its-Own-Backyard.aspx?Page=1