The method you call on the Geoprocessor object to execute the task depends on whether the task is synchronous or asynchronous. This information is available to you in the services directory page for the geoprocessing service, under Execution Type. The Execution Type is set when the model is published as a service. As a developer, you don't have any control over the type after it has been published.
Just to remind you, synchronous execution requires that the client wait for the results before continuing with the application code. In asynchronous execution, a client submits a job, continues to run other functions, and checks back later for completion of the job. By default, the client checks back for completion every second until the job has finished.