The hard-to-answer question is at what point it makes sense to go with VMware. Hyper-V 2012 leverages new capabilities in SMB 3.0 that give even the smallest shops the ability to stand up a high-availability cluster using low-cost servers and commodity SAS disk drives. Hyper-V 2012's host-to-host VM replication provides an additional level of redundancy not previously available from Microsoft and levels the playing field from that perspective.
At the same time, VMware has a similar function that uses the same Microsoft Volume Shadow Copy Service (VSS) under the covers. These and many other similarities make comparing the two products problematic. In the end they are both compelling products with plenty of capabilities that you can tweak to meet most any virtualization need.
Management toolsAt the low end, Microsoft gives you a basic set of tools in Hyper-V Manager, which comes as an installable option with Windows Server 2012. VMware's traditional management tool, the VMware vSphere Client, is a free client you must install on a Windows PC. Both offerings connect to remote hosts, allowing you to manage any system you can reach over the network.
Some functions are not possible in the basic management tools for either product. Advantage here goes to Microsoft as Hyper-V Manager can, for example, export a VM, then do an import to clone or copy the VM. With VMware you must be connected to vCenter Server in order to export or clone a VM. With respect to monitoring, however, the VMware vSphere Client provides much more information about both the host servers and the client VMs. VMware scores a point here for a more detailed graphical presentation.
VMware provides vCenter Server for managing large installations while Microsoft offers System Center 2012. The latest release of vCenter (5.1) adds a Web client to the mix, providing the ability to manage your VMware infrastructure from literally anywhere. Both VMware and Microsoft support automated management using Windows PowerShell. VMware offers a free add-on called PowerCLI that includes a long list of custom PowerShell cmdlets for managing your vSphere infrastructure.
Performance and scalabilityDeciding how to measure performance and scalability presents a challenge when comparing these two products. Microsoft has made a number of enhancements in Hyper-V 2012 that in some cases exceed the outer limits of vSphere. If you want to gauge scalability in terms of raw numbers like nodes supported in a cluster (64 for Hyper-V 2012 vs. 32 for vSphere 5.1) or VMs in a cluster (8,000 for Hyper-V 2012 vs. 4,000 for vSphere 5.1), you would deduce that Microsoft takes that round.