Grails unit test mock service vs assign service instance

What is the difference between mocking service and assign instance of class to service?

For example:

class MyService {

   def CallServiceMethod(){
     my business logic
   }
}

class MyController {
   def myService

   def callServiceMethod(){
     myService.callServiceMethod()
   }
}


@TestFor(MyController)
class MyControllerTests {

    @Before
    void setup() {
         controller?.myService = new MyService()
                    vs
         controller?.myService = mockFor(MyService)
    }

    void testCallServiceMethod(){
         controller.callServiceMethod()
    }
}

Any one help me please?