The answer is, as with most matters: It depends. Depending on the type of content you’re working with and the kind of information you’re asking for, it could easily either be one checkbox or two radio buttons. The collection of answers above can hopefully help you make a more informed decision. But as with all user interfaces, nothing beats the input you can get from user testing and research. So hopefully the answers above can at least serve as a starting point in situations where you need more to make a decision.