I recommend to use the deploy folder for -ds.xml files.
If you pack it together with your application you need to know about the final deployment during WAR/EAR creation time instead of decide during installation.
E.g. Test and production, you do not want to use the same database, right?
Indeed, I think it's best to let the configuration live inside the application server rather than being added to the deployment.
It's been frustrating finding out there are more than one way to deploy a datasource, when I can only find one way through the documentation.
There are often more ways to go,
and sometimes it is difficult to see the right one with the first try