It depends entirely on what you're trying to express.

Think about the semantics of the document. If the value is something 
which really is a single value _conceptually_, but which may at some 
point in the future want to carry additional sub-structure, make it an 
element. If it's a simple text value and will never be more, make it 
textual content, or -- if it's a characteristic of its containing 
element -- possibly an attribute value (leaving the possibility open of 
other attributes and/or content to be added later).

Look at other XML-based languages which solve similar problems and 
decide which make the most sense for you.

And, as with any design skill you're still learning, resign yourself to 
the fact that you're going to make some mistakes and have to do some 
redesign until you develop a sense of style and the ability to think in 
terms of larger patterns. Consider creating an initial version, working 
with it for a while, and deciding whether you like the way it behaves 
before you let it get cast in concrete.