AngularJS <select> issues

I'm having a weird problem. I'm using AngularJS in my app and I'm having issues with tag. I have a Controller to have cities and states handled by my elements:

function MeuController($scope, $http) {  

$scope.states = [];  
$scope.selectedState = '';  

$scope.cities = [];  
$scope.selectedCity = '';  

$http.get('/state/GetStates').success(function(result) {  
    $scope.states = result;  
});  

$scope.getCities = function() {  
    $http.get('/cities/GetCitiesByState?state=' + $scope.selectedState).success(function(result) {  
        $scope.cities = result;  
    });  
}  
});  

At this point, everything is OK and easy to understand. But...

When I create my elements this way:

<select class="span2" name="SelectedState" ng-model="selectedState"   
ng-change="getCities()" ng-options="state.ID as state.Name for state in states">  
    <option></option>  
</select>  

<select class="span6" name="SelectedCity" ng-model="selectedCity"  
ng-options="city.ID as city.Name for city in cities">  
    <option></option>  
</select>  

... my elements aren't filled.

If I try this way:

<select class="span2" name="SelectedState" ng-model="selectedState"   
ng-change="getCities()">  
    <option ng-repeat="state in states" value="state.ID">{{state.Name}}</option>  
</select>  

<select class="span6" name="SelectedCity" ng-model="selectedCity">  
    <option ng-repeat="city in cities" value="city.ID">{{city.Name}}</option>  
</select>  

Now the values are filled into the elements. Although, if I change the value of "selectedState" scope variable, my element doesn't "select" the right value?

Anyone knows why?

Thank you so much!