Just my few cents... Basically MVC/MVP are presentation patterns and they deal more with loose coupling of presentation or UI elements with presentation logic or control flow....
The following wiki article has brief narration on the above..
N-tier broadly represents separation of Presentation, Application (presentation or business logic layer or data access component layer etc etc) and Data Layer + Some other layers like services, infrastructure etc....
NOTE: In MVC though most of the time the view, the controller and the model are in the same physical location, but in enterprise applications this may span across machines and boundaries.
So, the question of when to use what is kind of blurred, because you almost always kindof automagically resort to n-tier by default (as best practices) and then as complexity increases you resort to separate the presentation from the presentation logic by moving to MVC/MVP pattern on the presentation front. Note the data model and the presentation/view model are two different things (though in most sample applications, they are depicted as one).
Hope this helps.
Rajesh Pillai, Oct 28, 2011