XML - Should I Use an Element or an Attribute?

From MSDN...

Elements are used to encapsulate pieces of data, and attributes are generally used to provide accompanying information about an element, rather than to encapsulate raw data itself. Whether you actually use an element or an attribute is up to the needs of your application.

Use attributes if your information requires data of a simple type and:

•Your information requires a default or fixed value.
•Your information requires data that is metadata for an existing element
•If the size of your XML file is an issue, attributes tend to take up fewer bytes than elements

The following list describes the key differences between elements and attributes from a schema perspective:

•A schema can define whether the ordering of elements is significant, but attributes can occur in any order.
•Elements can be nested with the tag, which means one and only one of the listed elements can appear.
•Elements can occur more than once, but attributes cannot.

Source:
http://msdn.microsoft.com/en-us/library/7f0tkwcx(v=vs.71).aspx