Best approaches for top seller lists

We are currently developing two sites for the same client, one is an online store, the other just a catalogue. For the store they want a top sellers list (top 10 in various categories) and for the catalogue a top 50.

For the store we will simply make use of sales data from their Point-of-sales system via csv import and calculate the top 10 for each category via a straight forward SQL Select query. However, we are a bit divided on handling the catalogue as there is no sales data available. One suggestion is to simply create an xml file that defines the top 50 items by id, on the other hand we have proponents that favour using the database instead…

Thus I would like to know the comments and opinions of other developers in industry. What solutions do you use and why? What are the potential pitfalls and what issues may arise at a later time, say when upgrades or modifications need to be made?

Manually defined by the client. They want fine grained control over the list seeing as some items are effectively the same, e.g. the hard cover and soft cover version of the same book.

We decided on going the xml route as this will offer our client the best flexibility.

huh? top 50 based on what?