If you're still unaware of what these two Anti-Aliasing methods actually do, bit-tech will help clear things up for you. In their latest article, they explain(and show) how these new AA methods used by Nvidia and ATI to improve image quality on transparent textures.
"It would simply be too inefficient for game developers to replicate some effects with raw geometry made entirely of opaque textures and polygons. Also, anti-aliasing the scene would also be incredibly taxing, too. As a result, they often resort to using transparent textures to add more detail within the polygons that lay down the frame work for creating a realistic scene. This is where transparency anti-aliasing comes into play."