AngularJS: Select not 2-way binding to model

I think you are probably using a child scope and don't know it. ng-if, ng-repeat, ng-switch and ng-include all create child scopes. Values on your scope are inherited, but if you change the value in a child scope it sets a value on the child scope and leaves the inherited value unchanged on the parent. Try using an object instead to hold your values and see if that fixes it. Since you are only setting a property on an object and not a value directly on the scope it will use the parent scope's inherited object and update the value.

$scope.data = {
    existingClient: $scope.clients.length > 0 ? $scope.clients[0] : undefined
};

View:

<select ng-model="data.existingClient" 
        ng-options="client.name for client in clients">
</select>

You can use the AngularJS Batarang extension in chrome to help debug your scopes.

This is also a solution to keep parameters into your $scope object:

controller:

$scope.scope = $scope;
$scope.clients = [];
$scope.existingClient = $scope.clients.length > 0 ? $scope.clients[0] : undefined;

view:

<select ng-model="scope.existingClient" ng-options="client.name for client in clients"></select>