Well, it can be a tough choice. The 25mm Snapshot Skopar has an angle of view of 82 degrees, while the 21mm sees 90 degrees (diagonally). I think anything much wider than 21mm is getting into weird territory, along with the fisheye lenses! I have a 15 that doesn't see much use, but not a 21. I do have a 22-equivalent in medium format that I like a lot, and the 25mm Skopar was most useful on a tourist visit to southern Spain. I had 15, 40, and 90 with me too, that time, and I essentially had to be forced by circumstances to use the 15.
Extreme wide-angle lenses demand extra work from the photographer; they can be difficult to use effectively. Wider is harder. In regard to the 25mm, I have to admit that I later got a 28mm lens that is coupled... Because I was starting a project of indoor environmental portraits, wide-open close-up, and felt better with RF coupling, and further because the camera I wanted to use has parallax-corrected framelines for 28. With the f/4 21mm, neither reason would be valid I think. Greater DoF, and an external viewfinder needed in any case.
You might do some searches in the RFF Gallery to seek out 21mm shots to look at, and also 24 & 25mm shots. Might give you a better feel for how they're each used. I even once went to the extreme of buying a 21mm viewfinder to look at the world, to check if I "needed" that angle of view. It turned out I didn't get the 21...
