I just answered your question on RIO.  Device Net is another communication medium I am not real thrilled about.  By ethernet I am assuming that you are referring to Ethernet IP.  That is pretty much the industrial protocol verson of Ethernet. With that out of the way - on to your question.  Device net was designed primarily for discrete components; however, it does work ok for a limited number of more complex components.  It also can provide diagnostic information that may not be available in a hard wired situation.  It does not work well for communications such as PLC to PLC where potentially large amounts of information may need to be passed back and forth.  Ethernet IP is much better at this task.  Ethernet IP is limited by the processor you are using as to how many drops it can connect.  It therefore would not be a good medium to connect a large number of motor starters together in an MCC.