title: $watchGroup vs $watchCollection?
body: <p>There're <a href="https://docs.angularjs.org/api/ng/type/$rootScope.Scope" rel="noreferrer">2 ways to watch a group of variables</a> in Angular. But what's the difference between them?</p>
  
  <p>They both seem to do shallow watches. Are there situations where one is obviously preferable over the other?</p>