If you ask which is better ODBC or XML? I'll say that for the Home version, they are equal.
For the Professional version of avast!, XML is missing some functionality (compared to ODBC), so switching avast! Pro to XML storage is not a good idea: ODBC makes it possible to store the past sessions in the Professional version - which is not possible with XML. Appart from this, Enhanced User Interface behaves strange when switched to XML storage - when a task is started, another instance of it appears in the list (of the tasks, not of the corresponding sessions!). Alwil did not explore the problem deeper yet, so it could be a bug or just something caused by the inability to store additional info in the storage. Right now, XML storage is currently considered just a backup option for avast! Home.
On contrary, to URL blocking (WebShield provider) switch to the XML storage allows a higher limit of characters. The path could have up to 1000 characters.