From a number of threads and talks, it seems that Esri has recommended that splitting servers is more efficient than using a larger server.
For example (2) 2 core, 4 GB servers typically are more efficient than (1) 4 core 8GB server.
However, just today I was reading the exact opposite in the new 10.3.1 documentation:
It's recommended that you use a few large virtual machines as opposed to many small virtual machines. For example, a single virtual machine with eight CPUs will perform better than four virtual machines each with two CPUs. If you're configuring failover, high availability, or separate ArcGIS Server sites for separate purposes, follow the recommendation above to determine the size of machines in each site.
Anyone willing to comment on this?