The S2 is based on the N80, while the D100 is it's own animal. The D100 is significantly, no, miles ahead of the N80 in regards to focusing and metering. I have both the N80 and D100 and in low light studio situations, the D100 will spend a lot less time hunting for your subject while the N80 is don't it's best R2D2 impression. The metering on the D100 is also significantly more accurate in back-light and low light situations.
The rest, resolutions, features and so are up to personal preference.
Oh, don't bother with AAs in the grip on the D100, they only last hours, the Nikon battery packs last all day. In fact, if I was to do it again, I wouldn't bother with the veritcal/grip, battery pack. I don't find it all that useful and even though I'm using it now, it may end up on EBay. By the 18-35 ED instead with money, with the $100 rebate, it's the deal of the week now. Do by at least two the the Nikon battery packs though.
If you have any questions, please let me know.
Joel
Model: Nicolette, photographed Yesterday with the D-100