J2EE design strategies recommend that all business logic exposed to EJB clients should be placed in Stateless Session Beans. See Wrap Entity Beans with Session Beans (http://www.theserverside.com/patterns/thread.jsp?thread_id=625) pattern for more information.
Message Driven Beans can be considered a variety of Stateless Session Beans and thus perform similar tasks. The difference can be argued lies in the Bean invocation. However, Object Oriented and J2EE design principles call for a clear separation of responsibilities between Message Driven and Stateless Session Beans. MDBs should serve as asynchronous message consumers. They should receive, process and parse the messages but should not perform any actual work. As discussed above, all the business logic should be located in Stateless Session Beans. MDBs should interact with specific methods from these Beans instead of implementing the functionality themselves. Note that MDBs should not interact directly with Entity Beans but rather allow Stateless Session Beans to do this as a part of a business logic method.
This technique strengthens and supports previously defined J2EE design patterns by containing business logic in one logical location. It also allows MDBs to become easier to implement and more lightweight and efficient by separating message parsing from business logic implementation.