On his blog, Keith Mayer wrote a very good comparison between Windows Server 2012 R2 with System Center R2 and VMware vSphere 5.5 Enterprise Plus + vCenter Server 5.5.
I would highly suggest you to read this article, it’s a good and well researched comparison.
Read more here.
Microsoft changed some OEM Licence policies with Windows Server 2012.
In the past it was possible to downgrade a OEM Licence from a newer Serverversion to an older one e.g. Windows Server 2008 to Windows Server 2003.
This is changed now. If want to downgrade your Server Licence you have to buy Microsoft licence contracts like Open, Academic or Volume Contract.
With this you have link in your Licence portal to order a Windows Server 2008 key and Installation Media.
Most of the Microsoft OEM (e.g. Dell or HP) will sale “downgrade Kits” beginning in Q1/Q2 2013. So if you buy a new server, you can buy such a kit. This “downgrade kits” are not the same like the downgrade that Microsoft offers with their licence contracs. It’s more like a new licence OEM to a much cheaper price.
In my opinion it will only be possible to buy this kits with new servers. So if you still have OEM licences from Windows Server 2012, you will not be able to buy the downgrade.
To learn more about the new contract restrictions please read the official Microsoft Licencing Documents.