This may be a bit more of an SO question, but...
If I'm writing a client-side application for OS X that's going to sync with an online MySQL database, is there any difference on if I have CD use SQLite or XML?
It depends, as XML is not an RDBMS.
SQLite allows the querying of relations. XML (and JSON) allow for the navigation of hierarchies. Completely different ontological mappings of reality.
If you need an RDBMS backend, SQLite is for you. If you need record storage without any sophisticated query requirements, XML may work, depending on your language.