Performance per watt
I saw that the SPEC organization just released a new benchmark SPECpower_ssj2008,which measures the power utilized by a server at various points among the load curve (idle -> 100% utilized). A quick scan of the initial results shows that most servers use roughly 60% of maximum power while idle.
From a power consumption perspective, it is way better to have 3 servers at 50% utilization than to have 5 at 30% utilization. Taking figures from one of the submitted scores, a server at 50% utilization might burn 234 watts while a server at 30% utilization might burn 210.
This gives 702 watts for 3 servers @ 50% versus 1050 watts for 5 servers @ 30%.