The Biogon is actually a f2.8 - not much of a difference. The prewar version is HEAVY with the scariest rear element ever. It just sits there out in the open! It is a much lower contrast than either the Nikkor or VC as well as much more sensitive to flare. Resolution is very good, i.e damned sharp!
The postwar version is lighter and has T* coating. Not as flare sensitive and, again very sharp.
There is a small shift in focus between the Zeiss/Nikkor's - particularly in close focus. In most instances the depth of field takes care of it though.
If you go to our Flickr site (see below) and look for the set tagged "Zeikon project" you can see some stuff done with these lenses.
Also. if you tag Flickr "Nikkor 35mm f2.5" and "Voigtlander Color-Skopar 35mm f2.5" there should be plenty of images from these two lenses to peruse. Not the ideal medium for checking resolution, but it will give you an idea.