I think everyone in this thread will vouch for the lens they own.
I just got the DA 21 so I can comment on it. I've used briefly the 15 and didn't like its size, plus its price put it off the line for me.
I got the 21 to work with the F50 f1,7 as a low light/travel kit (possibly with the M150 f3.5). The 211 is marginally smaller, but not that much smaller, than the F50:
I find that it is the contraty to what you imply regarding handling. It is extremely small but it handles well, the focus ring (with quick shift) falls under the hand easily, and it's easy to hold the camera+lens for a long period of time.
It creates a 3D effect that I did not understand before experimenting with the lens:
focus is extremely fast, extremely reliable, IQ is what you'd expect from a limited (pixel peeping might show perks of flaws of both the 21 and 15, but to me images look amazing and that's what counts). No visible CA, distorsion under control, just goodness. And it's usable wide open.
For my part, I have the Sigma 17-70 and at f2.8 it covers the "below 20 mm" wide end quite well. 21 mm is a very confortable wide FOV, 50 was too cramped for many shots.
It really depends on your style. What I want to expose is why *I* chose the 21, and show that I'm glad I got the lens. YMMV, if you shoot differently than me, you might prefer the 15, but if you share my opinion then you'll be happy with the 21.