I currently use a 105mm EL Nikkor lens for my 6x7 negs which I am happy with. The only problem I have is when I enlarge around two thirds of the neg only and this restricts the size of print I can make.
I am looking to get an 80mm lens for these occasions to enable me to make bigger prints. I have my eye on an f4 Schneider Componon S and an f5.6 EL Nikkon, both used.
I know the Nikon handles 6x7 just fine. The advantage of the F/4 is that extra stop. But you might want somebody to confirm it covers more then the rated 6x6
For a selective enlargement of a 6x4.5 section of a 6x7 neg, both lenses would be fine. I use El-Nikkors, as already mentioned, having a max. aperture of f4 might be handy for doing very big enlargements as long as your negtive is totally flat, otherwise virtually no practical difference between the two lenses you mention.
Why not look at the Meopta Meogon 80mm lens. I can remember reading a magazine article from europe a ways back now regarding a comparison test done alongside Componon's, El Nikkors, and the Meogon smoked all of them across the board and at most apertures. They are a 6 element f2.8 design that are wonderfully sharp, contrasty, and relatively inexpensive.
Only saw your post now. I went ahead and orderred the Componon as it was cheaper than the Nikkon but in just as good condition according to the seller (Ffordes). I have purchased quite alot of gear from them over the years and have never been disappointed.