XMI is EMF specific format, that is only supposed to be edited using the designated tools like Designer. No automation there will be provided. However, DDL is just plain text.
We are planning to deprecate XMI altogether, in next major version of the Teiid Designer, it is all going to be DDL. The support for DDL based VDBs in Designer is also coming in the current versions of the Designer. In Designer you can (already|coming very very soon) find way to export the XMI based VDB into a DDL based VDB. We are talking further simplifications too. So, if want to participate, look at the Teiid Designer JIRAs.
So, I recommend just keep using the DDL, that is the path into the future. Yes, version control, collaboration, offline view, programmatic ability to create VDB, simple editing, defining of metadata using closest domain language, ability to other tools all are advantages.