I recall a comment somewhere by Galen Rowell either on the 24mm or the 20mm. The extra stop made the lens bigger, heavier and more prone to flare. For landscapes, it doesn't buy you much anyway, as you are more likely to be stopped down. What is the application you have in mind?

Here's the link I was thinking of:
http://mountainlight.com/rowell/gr_camera_bag.html

Matt