You simply need to explain to her the difference between 16:9 and 4:3. A really good way to do it is to get both widescreen and fullscreen versions of a DVD and show a scene. Let her see how they crop the edges (or do pan and scan) to get it to "fill" the screen.
Then explain to her that the question came down to which will you be watching more of - 16:9 content or 4:3 content. The format of the set you buy should match the answer to that question. -- Brian
