I own an E-10. Love the image quality, hate the write speed. I am shooting mostly concerts and other live events these days and the slow write times and the top ISO of 320 are limiting.
Anyway, a camera store local to me has several used S-1s in practically mint condition with a 90-day warranty for a reasonable price. Alot less than the S2, which I just can't afford. Like you I have tons of Nikon 35mm equipment so lenses aren't an issue.
The S1 users I have talked to all like their cameras. Noise doesn't seem to be a problem at ISO 320...they say less than the grain you would see even with the better ISO 400 films. I never found the noise on my E-10 at ISO 320 to be objectionable. Sometimes a little more in the shadow areas, but that is the price you pay for higher ISO setting, either in the film or the digital world.
In my opinion, based on what I have been told, it is a good step up. I won't be trading my E-10, it has served me well. But I do think I will be making that step up soon...