$watch vs. ngChange
Bottom line - You can achieve with $watch every thing you can achieve with ng-change but not vice-versa.
Purposes:
ngChange - binded to a HTML element
$watch - observing scope's model objects (HTML object models included)
My rule of thumb - if you can use ng-change use it to match your scenario, otherwise use $watch
Why you shouldnt use $watch?
- It’s inefficient - Adding complexity to your $digest
- It’s hard to test effectively
- It's not clean
You have it mostly right. ng-change is very DOM specific and for evaluating an expression when the change event fires on a DOM element.
$watch however, is a lower-level (and more general purpose) utility that watches your view model or $scope. So your watch function will fire every time the user types a key (in the example of an input).
So to contrast, one listens to DOM events, the other watches your data.
$watch adds more complexity do the $digest, making it less efficient. In your case ngChange it's a cleaner and easier solution...