For switchless RS/6000 SP systems or SPs with the newer SP Switch, the decision will be based on a more functional level.
Event Management is much more flexible in HACMP/ES, since you can define custom events. These events can act on anything that haemd can detect, which is virtually anything measurable on an AIX system. How to customize events is explained in great detail in the redbook HACMP Enhanced Scalability,
If you have an NFS server that you need to make highly available, especially if it is heavily used, and NFS file locking is a major issue, you will need to run HANFS for AIX.
