When comparing Hyper-V Server 2008 to ESXi 3.5, it’s interesting to note the feature gap between the two products. Especially when it comes to high availability: Multiple boxes with ESXi can be used together with Virtual Infrastructure to form a high available virtualization platform with VMotion capabilities. Hyper-V Server 2008 installations can not be used in a failover cluster.
Hyper-V Server 2008 R2 offers extended functionality beyond the currently released version in terms of system capabilities and installable roles. The screenshot illustrates the maximum number of processors, maximum amount of RAM and maximum amount of virtual guests you can run on both versions of Hyper-V Server: