I compared the FA 20mm f/2.8 with DA 21mm f/3.2 by taking a simple photo of an indoor area.
A room lit by a fluorescent light
Fully manual mode, so that shutter speed is the same for a given aperture.
JPEG conversion with Photoshop CS3 using same parameters. Best quality JPEG.
Below is the area photographed.
This one is with FA 20mm at f/3.2, 1/10s.
With DA 21mm at f/3.2.
What I noticed in comparing the two lenses:
1. There is difference in resolution (see Photozone.de for the comparison). But it's hardly noticeable unless you are counting the pixels.
2. FA 20mm has more accurate color. The room was lit with a fluorescent light and the pictures were converted with that setting.
3. FA 20mm shows more detail in the shadows. See the dark area on the right side (outside the door which was lit with a dim light.)
But the DA 21mm is a nice small lens. It'd be great with a small and light camera such as *ist D or *ist DS when traveling.