This week's giveaway is in the EJB and other Java EE Technologies forum. We're giving away four copies of EJB 3 in Action and have Debu Panda, Reza Rahman, Ryan Cuprak, and Michael Remijan on-line! See this thread for details.
As far as i know, the applicationContext.xml is a Spring specific file. So it's Spring which is responsible for creating and managing the datasource. But if you want JBoss to create the datasource and manage it, then *-ds.xml files are the only option.
Haven't used Spring, so I don't know what advantages you might see when using Spring datasources instead of JBoss'