Both database and XML have got their advantages and dis-advantages.
1)Migration to a different server is easy. You need only copy a set of XML files.
2)reduction in cost - no need to buy database
1)You cannot make a schedules backup of data etc.
2)It is difficult to have two different version of content.
1)You can backup database on a fixed schedule.
2) security of data is more
3) version control of the content is more easy. You can put the content for next week in the same table and can make your script to show it only on next week. You can do this with XML also, but with more difficulty.
1) It is difficult to migrate to a different database.
2) cost of hosting will be slightly more as data enabled hosting facilities are generally costlier.
But, IMO, clients are generally not technology specific unless it affects their budget very much. Once you setup a CMS with database and scripts, why do they want to migrate to different technology frequently ? That is why I suggested database.
If you want to make frequent technology changes (or if you are building a general CMS which may be supplied to a large number of clients), then it is better to go for XML.