Originally Posted by Quicksilver

Of course the decision is subjective based on your needs. However;
The Extreme will performed twice as fast as the Express before you enabled 802.11n.
After you enabled 802.11n, the Extreme will run almost 5X as fast (4.6X to be a little more precise).

The Express will allow you to connect up to 10 computers, however has a shorter range than the Extreme (this may or may not be an issue for you). Other than that the only difference between the Express and Extreme is that you can connect speakers to the Express and stream iTunes music to them.