Since default values are only run at creation time, where for all intents and purposes there is pretty much no information on the work item, you want to use a calculated value. Consider what information is available when a work item is created:

• Creation Date
• Creator, maybe
• Owning project area?
  

All the other values such as Type, category etc and especially your custom enumerations only become available after the creation of the work item, when chosen by the user.

So you will likely have to use a calculated value. You will have to find out how to deal with the "calculated default" and the situation where users add their own information.