I have the CV 21/25 chrome finder which I think looks much nicer than the 21 or 25 plastic finders, especially on a chrome body. Image view and functionality are pretty much the same.
I do use a Leitz 3.5cm SBLOO and it's awesome. I use it on Leica Barnack bodies, a Bessa T and a Contax IIA, and it's really bright with decent eye relief. No issues with flare that I have ever encountered. A bit bigger that the equivalent Cosina, but it's just about perfect. If you find a clean one at a fair price, grab it. Good Luck!