When using unit testing to develop something in an MVC style architecture, what components do you actually test?

Say if you've got an ArticlesModel, ArticlesController and ArticlesView, do you write seperate tests for these (TestOfArticlesModel, TestOfArticlesController and TestOfArticlesView) or one all-encompassing test TestOfArticles?

Right now I'm testing them all individually and I seem to be repeating myself a lot, which smacks of inefficiency...

Any help is appreciated!