Not many of either made in Nikon S-mount I think, might be a little hard to find. I have both in Leica thread mount. The Skopar got off to a bad start, as someone spread the word that it was "soft", not particularly sharp, and this was repeated far and wide by those who didn't have one. So it had a poor reputation early-on. I took a chance on a used one at about half-price and didn't find it "soft" or have anything else to complain about... except a small amount of pincushion distortion. Erik van Straten uses one in Nikon mount I believe, likes it a lot and doesn't see any distortion in his.
The f/3.5 Heliar came along later, and I have to say the non-collapsing Nikon mount version is pretty strange looking. But, having the same optics as the Leica screw-mount version it should be an equal gem in use. And, not collapsible, it might well have better ergonomics.
I am not particularly sensitive to the differences in rendering of various lenses, though I have to admit there are some that just seem to have something undefinably extra. This f/3.5 Heliar is one of them I think. It's a traditional Heliar 5-element design, while the Skopar has 7, surprising for such a modest aperture. IMO the Skopar is an excellent lens too.