No question in my mind that the 21mm would be my next choice. Ultra-wide angle suits the kind of photography that I like. Even though, I use the 21mm sparingly, it is amazing to me how many of my best photos were taken with it. In case you're wondering, I use the 35mm and 45mm lenses for most general photography.
the best explanation i've heard (lifted from www.dantestella.com) is this:
35mm is both eyes open, glancing at what you see. 45mm is one eye open. 90mm is the detail you see with one eye open. 28/21mm i have no idea, i don't 'see' that wide.
if you're hunting detail, check out the 90mm. if you are seeing things with slightly more detail than the 35mm but still not 'wide' you are looking at a 45mm(actully a 48mm ) planar. if you are a horse, you'll be happiest with the hologon!
On a somewhat related note, is there any way to determine by looking at an image at what focal length it was taken? For ex&le, if a wide angle lens was used, the perspective seems more natural if the photo is held closer to one's eyes.
Obviously the compression and lack of DOF associated with a telephoto stands out, but the difference between a 20MM, 24MM, 28MM, and 35MM (for ex&le) does not always seem readily apparent (especially if the photo may have been cropped).....or perhaps it is all just intangible experience??