content_url(); seems to be the best option in your case. You want the url of wp-content folder, there is no reason to use other workarounds. Also, accroding with codex, content_url() takes care of current site in a multisite network.
But in your example you are using it a little wrong; content_url() doesn’t accept a second parameter. It should be content_url( '/uw/test.css' ).