This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
I think difference lies in the way they get input content instead of the return type. Servlet gets only HttpRequest/HttpResponse objects where as for REST method you have control how you will provide input to it..