Hyper-V is a bit of a pain as it doesn't let you over-commit on memory.
VMWare ESXi looks like a much better bet - I'm currently thinking about moving 40+ VMs in a development environment from Hyper-V to ESX so we can run more of them at the same time.
Actually, with Windows Server 2008 R2 SP1 it will add dynamic memory which is the same as VMware's memory overcommit as well as RemoteFX for 3D graphics display for virtualized desktops.
VMWare ESXi looks like a much better bet - I'm currently thinking about moving 40+ VMs in a development environment from Hyper-V to ESX so we can run more of them at the same time.