Both the DA15 and DA21 are excellent, but very different lenses.
The DA15 has low distortion - so low that I've never really corrected for it, as far as I can remember. It's a superb rectilinear wide-angle, and rightly has a lot of fans. I don't shoot mine very often, but I'm always impressed when I do.
The DA21 has (of course) a
much narrower field of view. It exhibits a certain amount of fairly mild and simple barrel distortion, which is easily corrected internally for JPEG, or in post-processing if you're shooting RAW. It's very popular, and one of my all-time-favourite lenses.
As I said, these lenses have very different fields of view, so they arguably suit quite different use-cases. It's down to what you intend to shoot, and how, as to which would be best - but they're both great