My guideline is if something feels like a "header" or like "meta-information", it goes into an attribute, if it feels like the actual information, it goes into an element. Totally subjective, obviously, and a very flexible rule
There are technical considerations as well, of course - some of the things that can go into a CDATA section can't be put into an attribute. And elements have an ordering, while attributes don't.
Since you're asking in the WS forum,
this thread might be of interest to you.