merge vs. update

saveOrUpdate() does the following : 

• if the object is already persistent in this session, do nothing. 
• if another object associated with the session has the same identifier, throw an exception. 
• if the object has no identifier property, save() it. 
• if the object's identifier has the value assigned to a newly instantiated object, save() it. 
• if the object is versioned (by a or ), and the version property value is the same value assigned to a newly instantiated object, save() it otherwise update() the object.

merge() is very different :  

• if there is a persistent instance with the same identifier currently associated with the session, copy the state of the given object onto the persistent instance. 
• if there is no persistent instance currently associated with the session, try to load it from the database, or create a new persistent instance. 
• the persistent instance is returned. 
• the given instance does not become associated with the session, it remains detached.

So, Use : 

• update() : if you are sure that the session does not contain an already persistent instance with the same identifier.
• merge()  : if you want to merge your modifications at any time without consideration of the state of the session.