For some strange reason Microsoft exams prefer the first.
However I would recommend the second. This is far easier (via a subclass) to create a proxy that explicitly requires the url to be specified. This came in handy on a project that communicates to many instances of a second app via web services. We have the one web service that is instantiated at a number of locations. By parameterising the web service we are able to allow the user to specify the location of the web service and to choose between them at run time.