I've used both the CV 35mm f2.5 (the non-pancake version) and the CV 35mm f1.7 and briefly owned both lenses at the same time. Therefore, I was able to carry out a comparison test of my own. I put some of the shots from my test on my gallery for RFF some months ago. The link is given below.
Nothing much to choose between them. The OOF (bokeh) effect is slightly better on the f1.7. The f1.7 would also be more suitable if you are likely to use the lens for portraiture photos at or near maximum aperture.
The f2.5 lens is half the size and weight of the f1.7 and also has a minimum aperture of f22. The f1.7 lens only goes down to f16. For image quality at mid aperture, I would say the f2.5 has the edge.
In case you wanted to know... I sold my f2.5 and kept the f1.7. The reasons are the f1.7 OOF effect is much nicer and its easier to focus.
There is a good lens test report here
It is intended for Epson RD-1 users, but is quite useful to us 35mm ludites.
Voigtlander Bessa R2A, CV lenses: 25/4, 35/1.7, 50/2.5, 1936 Leica 9cm f4 Elmar
Leica R7, 50/2 Summicron, 90/2.8 Elmarit, 180/3.4 Apo-Telyt-R
I'm not going digital... It's not photography, its computing!