I would only use SQL if you are in a big enterprise environment (High security, lots of IT-control, Test environments, etc).
Otherwise just use XML. Reading/Writing to XML is fast enough.
It's not a performance choice, it's more an interfacing/control choice.
I've implemented a lot of QlikView servers and I always use XML.
Here's what the Server reference manual says about it, sounds like a good starting point.
BTW, only SQL Server is supported.
Database requirements and recommendations for QlikView Publisher
The database in QlikView Publisher can be either a Microsoft SQL Server or an XML repository that
requires no pre-installed software.
The supported versions of Microsoft SQL Server are SQL Server 2000, SQL 2005 or SQL 2008. If you
have a Microsoft SQL Server already set up we recommend using that. The XML repository is sufficient for most installations when it comes to performance.
If you do not have a Microsoft SQL Server available we recommend that you start with an XML
repository installation and upgrade to Microsoft SQL Server if the performance is insufficient. It is possible to migrate all data in the database between XML repository and SQL Server.