I have both. I prefer the 24mm most of the time. This is partly because the finder for the 24mm is easier to use and frame with, compared to the 21. I have the Leica 21mm and 24mm finders, and the Zeiss 25/28mm. The framelines on the 21mm finder actually are wider than the hard plastic edges of the viewing field. On the 24mm Leica finder, they are inset from the edge, and easier to use. The 25mm Zeiss finder is accurate enough, and the bright lines are entirely straight, making it easier to line up the verticals in architectural shots. On a trip to San Diego, I was actually able to shoot the 24mm without the finder, just using the M6 0.58 finder--and with good results!
So the 24mm is easier to use. But it is also simply wide enough for most shots. I never really need the extra coverage of the 21mm. So if one had to go, I would part with my 21mm.