It can be used as alternatives. In MSBuild you can use attributes and sub elements interchangeably. Which, if you're writing it, gives you a choice of preference. I typically prefer attributes for conciseness (vertical density), but switch to subelements once the length/number becomes a (significant) downside.

Of course that's more of a human writing view. Your point about ambiguity in de-/serialization still stands at least until the interface defines expectation or behavior as a general mechanism one way or the other, or with specific schema.