isthewebsitedown if you are asking, probably not. if I am asking, probably so

Hyper-V R2 Vs. VMware vSphere 4.0

Windows 2008 R2 is available now and one of the 4 key points of improvement is in the area of Microsoft's HyperVisor, Hyper-V. What has been lacking until recently is a side by side comparison of the features and limitations of Hyper-V R2 Vs. VMare vSphere 4.0. The killer feature that everyone has been waiting for on Hyper-V has been the live migration, designed to compete with vMotion. The devil is in the details though, and there are lots of details about this feature and others that factor into the decision as to which product is better for your environment. CTI has a great white paper that discusses the differences. You hit the side by side comparison on about page 17 of the paper (no reg required). Bottom line: here are the top 6 differentiators as far as I am concerned:

1. Memory optimization: VMWare has over-commit protection. Hyper-V really doesn't. It can reserve RAM, but it is pretty hokey compared to what VMware can do. Who cares right? RAM is cheap! Not so fast. Some RAM on these higher end virtualization and blade servers can break the bank.
2. Live migration: VMware wins here too. Hyper-V R2 has it, but it can only do one machine at a time, and the way it handles shared storage is weak. VMware has been at the game longer and I expect that this gap will be closed but for now it could be a dealbreaker for those who load their hosts down.
3. Guest support: Another score for VMware. The only non-Windows guest supported on Hyper-V is Suse Linux. Not a huge deal if you are running Web servers I guess, but those guest tools start to get really nice if you are needing to do any of the more advanced functionality. VMware supports most *ux's including SCO OpenServer, SCO Unixware, Free BSD, Debian and CentOS. Again, not necessarily a deal breaker.
4. Ability to hot-add disks: VMware can add them easily. Hyper-V can only add virtual SCSI devices, not IDE
5. Number of guests: This one is a mix. Hyper-V can have 512 loaded, but only 192 running ("only," he says). VMware can run 256 at the same time, with up to 8 virtual CPUs and 255GB of virtual memory, compared to 4 CPU's and 64GB on Hyper-V.
6. Monitoring: Hyper-V wins here. Since it is based on a Windows Core box that is joined to the domain, you can capitalize on the tools built into Windows, which are legion.

So who wins? Honestly, it depends on who is judging. For implementations of 30 Windows virtual servers and under, I don't see why you would pick VMware, honestly. It comes out cheaper by most people's math, you have fewer vendors to beg for support from, and you have fewer new interfaces to learn. For larger implementations, it depends a lot on how heavily you intend to stack the VM's on the hosts and what kind of downtime you can tolerate should one of the hosts fail.

As far as dollars are concerned, most calculators will show that the initial cost for Hyper-V is cheaper for similar implementations. Most, of course, except for the one that VMware provides (big shock).

Dollars for initial implementation are small potatoes though, compared to supporting a poorly planned implementation. It is always going to be good to bring in an experienced party that can help guide you through some of the pitfalls.