Hi
Last year I have been comparing the 47mm Digitar to my 35mm and 60mm HR Rodenstocks on my ALPA SWA with a P45+ back.
The Digitar has a much bigger circle of image and costs less. Although it would fit the gap perfectly, I ended up not buying it:
There's quite strong CA towards the edges and some vignetting (I didn't have the center filter): Both can esily be dealt with in post, though, and sharpness and contast were very good.
I assume with the new version of C1 (4.5), you could easily correct CA automatically as well as vignetting and get absolutely fine results.
You will however always have to make the lens cast file.
Another small disadvantage is the maximum aperture of f5,6: The images are a little harder to see on the groundglass, compared to f4 on the HR's.
I also use the Digitar 180mm with great success and therefore would expect the 90mm to be of very high quality as well.
If you have them offered at a good price, it sounds very tempting!
Hope that helps
Jost