As a knowledgeable hobbyist, I assume you know that 1:1 true macro shots are best achieved using dedicated macro lenses like Nikon 60mm f2.8 or 105mm f2.8. However, you can also use relatively inexpensive closeup / macro lens attachments but the results are bit of mixed bag with vignetting and loss of edge sharpness, etc. but nevertheless, it might give you good enough result, depending on your IQ requirement.
As for the "macro" feature on P&S cameras, none can reproduce real 1:1 macro like a dedicated lens can. Both S100 and G12 can do "macro" but most all P&S now days have that capability; no big deal. So I'm not sure if that should be the compelling reason for deciding one over the other.
I like the feel and the build quality of S100 but I don't like the lack of buttons and menu driven features. But it is very portable. Both S100 and G12 have so called "larger" sensor (1/1.6 versus 1/2.3) for "better" picture quality, but when it comes to IQ difference and high ISO capability, it is not as pronounced when comparing the two sensors. It makes much bigger difference when comparing D90's APS-c sensor size versus G12/S100's 1/1.6 sensor. That's why I chose Canon 300HS ($129) over S100 and use the difference to purchase P7100, which is very similar to G12 in features and capabilities. I too am a Nikon DSLR user but I generally like/prefer Canon P&Ss. But I chose P7100 over G12 since I can use Nikon's external flash on P7100. P7100 also has very useful 7X zoom. Although I give the ergonomic edge to G12, IQ is very comparable between G12 and P7100. Both P7100 and G12 has articulating screen but different designs. G12 flips out to the side, P7100 flips out and down or up. You may like one design over other.
