24mm is probably my favorite focal length, so I am a little biased. I know I need one, though I have no way to know if you do. My answer would be, you need both 24 and 28 (and 35); but not everyone needs the extra expense and complication of carrying both.
Think of a lens, not as an individual lens, but as part of a system of lenses. If you already have a 50, and want to add one lens, a 28 makes sense, as the gap between them is not too wide. But if you have a 35, then a 24-35-50 outfit would be more versatile than 28-35-50. So think ahead to the outfit you want to build to.
It was not hard for me to use the 24mm with the built-in 0.58 finder, until I got around to buying a finder for it. The framing was not highly accurate, but with a little judgement I did well. Both the 24mm Leitz and the 25/28 Zeiss are good--well, the Zeiss is very good! It is bulkier though.
Well, take your time, as part of the fun is in the planning . . .
Best, Rob