title: Change rootViewController vs. present ModalViewController
body: <p>I'm having difficulty thinking through the design tradeoffs of changing the root view controller of an app vs. presenting the new view controller modally.</p>
  
  <p>In which situations should I change the root and in which should I present modal?  I'm looking for general app design guidelines.  Should I always present modal and only establish the rootViewController once (in the appDelegate)?</p>