Leica lenses made in the 50s and 60s are of an entirely different quality to those made today. Not to say anything bad about modern lenses, but they don't hold a candle to the classics.
Optically I think the biggest difference you will see from these two lenses is in the amount of contrast. Both are very sharp, but the Summaron draws a very "old fashioned" looking image, lower in contrast than the modern Skopar. You will find very few negative comments about the Skopar it is an excellent lens. It does offer high contrast though and on an R-D1s a lower contrast lens is more desirable no?
I personally really like the images I see taken with 35/2.8 Summarons. They generally sell for @$500 US. If you can find one for the price of the Skopar it is a very good buy if it is good condition. I am sure if you go to flickr and search tags for Summaron you find a number of photographs taken with this lens. If so, you can see if it produces anything particularly attractive to you.
Also, I purchased a Canon 35/2.8 which produces images similar to the Summaron. It can be found though for less than $150. A Jupiter 12 would be similar too, but not a fit for your R-D1s due to the large rear element.