Rob, that is a tough call. I've got them both presently, along with the 28mm 'Cron. Edge distortion on the 21, as you would expect, can be more extreme depending upon subject placement. I personally use an external view finder most of the time, but I find that to be an asset more than a detriment. Especially using the 21mm. The 24mm I can get away just using the edges of the viewfinder frame, but as already mentioned it is not the most accurate by any means. I think you will want the external finder with either of them.
That said, the image quality from both lenses is excellent. Sharp, even wide open, in both cases. Micro contrast is over the top great. Close focus distance is about the same. They are so close in focal length that I would suggest you probably will not need or want both. So my suggestion would be to decide what other lenses you plan to use and then decide which one to choose. In my own case, I own and love the 28mm, so keeping the 21mm makes more sense for me.
If I was shooting mainly a 35mm, and did not plan on buying a 28mm, then the 24mm probably makes more sense. The 24mm and the 28mm are so close in focal length you won't want them both. But the gap between 24mm and 35mm is just about perfect in my experience.
Since both are so wide, think in terms of pairing would be my suggestion. 21 & 28, or 24 & 35 make the most comfortable pairs I find for me. In either case, whichever choice you end up making you can't go wrong. Both are superb optics.