vSphere 4 offers 30 % greater consolidation.

If in your existing ESX Server infrastructure you were able to achive some consolidation ratios. Then if you’re going for the vSphere this year you’ll be able to gain 30% more consolidation ratio…..

… So if you’re an existing customer today and you have a 100 host deployment using our vi3.5 product, simply upgrading the software will save you $2 Million dollars a year

VMware:  Virtual Reality blog has more details about the density of VMs.

Using VMware you’ll need less physical servers to buy. The same for Floor space and network equipement, storage (Thin Provisionning in vSphere). Also less power and cooling to support your virtual infrastructure … Also you’ll have more advanced functionality like VMotion, DRS, HA, etc.