A choice between a node and an attribute is not always clear, but there are helpful guidelines out there. A search for xml nodes versus attributes will give you a list of articles to ponder -- sorry for not being more detailed, but I am away from my archives, or else I could have given you links to the places that helped me to visualize it better. Here is one classic, though, from as far back as 1992: http://xml.coverpages.org/attrSperberg92.html

EDIT: ok, the first link is to an IBM Research article which I recall having been useful: http://www.ibm.com/developerworks/xml/library/x-eleatt.html