The IIIS is smaller than the IIIL by a good bit, but larger than the CineDigitar. All 3 have adjustable astigmatism. The IIIS and IIIL are made the same way internally I am told by Isco, the IIIS is simply smaller. The IIIL lists for about $800 less than the IIIL. MSRP around $5600-$5800. Street varies by dealer, and whether or not it is bundled with a CineSlide transport or IscoScopeII stand.
I have had a IIIS and "played" with it, but I do not have one in stock. Takes a couple of days to get them usually. I also have not had the RS20 in hand yet. Since the CineDigitar is reputed to be "close" to working with the RS20, my expectation is that the IIIS would work fine with the JVC, however that is a hypothesis. My personal preference is the Isco over the CineDigitar. Same company owns them, yes, however that is at a high level. They are still made at two entirely different locations by two seperate companies. I expect we will see some synergies develop in the future, but for now, for practical purposes, they are different products. For the Isco, and not for a profit reason, my recommendation continues to be the IIIL due to it's long term flexibility and resale value. It won't matter what PJ you own in 2 years, you will never have to change that lens. You could easily need to change the others. I posted a thread with photos on the IIIS here on AVS back when I had the pre-production unit in hand.
They can be purchased direct from TechHT, from dealers, or if you are an existing AVS customer, Jason here at AVS may be able to assist with better than MSRP pricing direct or via referral.
Hope that helps,