What does a 2U server give you that a 1U server or a blade doesn't ?
That aside, a VMware-licensed OS X is clearly the best and most obvious solution to the discontinuation of the Xserve. Virtualisation is unquestionably the future for the vast, vast majority of server applications. The benefits are simply to huge to ignore.
The list of things VMware does poorly is vanishingly small. Heck, even if you only have a single server, you're nearly always better off running it as a VM on the free ESXi product, rather than direct on the hardware.
The biggest benefit from virtualisation is in manageability, not density. Most datacentre facilities barely even let you max out your racks with 1U servers, let alone blades, because they can't provide sufficient power and cooling.