It depends on a design. There are no rules for URIs at REST over HTTP (main thing is that they are unique). Often it comes to the matter of taste and intuition...
I take following approach:
- url path-element: The resource and its path-element forms a directory traversal and a subresource (e.g. /items/{id} , /users/items). When unsure ask your colleagues, if they think that traversal and they think in "another directory" most likely path-element is the right choice
- url parameter: when there is no traversal really (search resources with multiple query parameters are a very nice example for that)
