Hi Kathy,
You read my mind. I was going to post this question before. But since you've started, I'll just join in.
To start, I own FA*24/2 and almost bought a DA 21/3.2 2 days ago.
I didn't get the DA21 yet, but maybe someday I'll get it just to compare both of them.
What I like w/ my FA*24 is that this is the only wide angle prime lens in Pentax that is fast enough for dim light. It's almost 1.5x wider than DA21.
I don't know how it performed when both tested at f/3.2 but I guess FA*24 will win since it's stopped down.
Also the contrast produced by FA*24 is amazing. Sometimes when I missed the focus, the picture still seems sharp because of the contrast. I think this is another good point of this lens. But again, I wish I have DA21 too for comparison.
As Peter said, the DA21 's advantage is the small size, but if you need it for low light shooting, you'll need to add flash anyway (which adds to the weight and size). For casual need in good lighting condition, I agree that DA21 is more convenient.
My other reason to keep FA*24 is the unique feeling of owning one of Pentax's best lens and satisfy my full frame dream
I don't ask Pentax to make one but in case they do, this lens will outperform DA21 for sure since it's designed for full frame anyway.