I own an original Zeiss Biogon 21/4.5 from the 1950s, along with a CV25/4 and a Nikkor 28/3.5.
I usually find 21mm to be a bit too wide for my taste. That's why I bought the CV 25mm a year ago, and it's a great lens. When I did newspaper work, I always found 20/21 to be a little too wide, so that I'd often crop out some of the most distorted areas along the edges, whereas I can shoot a 24/25 all day long. (I don't understand the above comment about the Nikkor 24/2.8 which is one of my favorite SLR lenses ... very wide angles, due to their perspective, have distortion on the edges, and an SLR gives you more control over it). Anyway, the 25/4 is a great lens. I really like that it is RF coupled, and the finder that comes with it is excellent.
I tend to use the 28/3.5 Nikkor the most, because I like that focal length. I use the 35/1.8 for very low-light work and the 28mm lens for the rest of my shooting. It's a very comfortable focal length, works well with the SP and S3 without a separate finder. I assume the more modern CV lens would have more contrast and more "pop" to the colors. That's the difference between my 28 Nikkor and 25 CV lens.
The 35, 28 and 25mm lenses are both very small and very light, so I tend to carry all three. For walkaround lenses, I usually leave the 25 at home and carry the 28, 35 and 50. My travel anywhere kit is the 28, 35, 50 and 105. My photojournalism kit is two bodies and the 25 (formerly the 21), plus the 28, 35, 50, 85 and 135).
At this point in my life, my photography is mainly a family hobby. If I were doing more shooting work on a regular basis, I'd almost certainly get the 15/4.5 CV lens. In my SLR days, I used a Tokina 17mm lens and got a number of great shots with it. When you need wide, you need wide.
Attaching a couple of recent shots from the CV 25/4.
