Hi Tom, I think this might be what you're looking for: http://www.ibm.com/developerworks/bpm/bpmjournal/1503_simmons/1503_simmons.html
The author lists out 5 criteria to use when determining to externalize. Just scroll down to the 4th black heading. The criteria include complexity, life cycle management, reuse of decision services, business and IT collaboration, and decision services management and visibility.
I hope this helps.