Though the first approach is advocated as 'the angular like' approach, I feel this adds overheads.
Consider if I want to use this myservice.foo function in 10 different controllers. I will have to specify this 'myService' dependency and then $scope.callFoo scope property in all ten of them. This is simply a repetition and somehow violates the DRY principle.
Whereas, if I use the $rootScope approach, I specify this global function gobalFoo only once and it will be available in all my future controllers, no matter how many.