S1 scopes are multicoated (multicoating on ocular and objective lenses and single layer coatings on the rest of the glass).
S2 scopes are fully multicoated throughout. They hold POI very well. IMO, S2/S3 holds POI as well as any other scope available at any price. S2/S3 also have some of the best W/E adjustments out there. I have one S1 scope and it holds POI just fine, but I've not had it on anything that kicks harder than 8mm Mauser. On the mauser it worked very well and POI never shifted.
They are made in Japan.
I think that S1 and S2 scopes are the best deals available out there in their price ranges.
What is your budget? What features are you looking for? What kind of a gun is this going to go on?