They are different cameras. The S95 has a different lens. It has the same sensor (or is it the S95 and the G12 are the same?). The S95 is a smaller camera and takes smaller housings which is a good thing. From what I hear, the G12 is better at macro. I have the G11 and in order to do macro, you have to be in the wide angle range. That means you have to get REALLY close, which is not good. The camera does work well with an Inon wet lens. The limiting part of the G11 is that the wide angle is a functional 35 mm underwater and that is not wide enough. You need to get into one of the high end housings to be able to fit it with a dome that gives you true wide angle. Without a wide angle situation, I often found myself having to back off too far from large subjects: stingrays, turtles, divers, large grouper and landscapes to allow the strobes to adequately light them and to produce sharp photos because of the stuff in the water column.
As for strobes, it seems to me that the S95 is more limited with strobes but I am not sure.
Now, Canon has come out with a new version in its G series. The camera has a far larger sensor which puts its sensor nearly on par with a DSLR. Of course, the camera costs quite a bit. I have not heard of any housings for it yet but I am sure they will be out soon.
Now as I understand it, the S100 is more compact, takes a much more compact housing, does RAW (vital). I do not know if it will take wet lenses. I think if you want to do decent macro, you need the wet lens. Also, you will find yourself limited if you do not have a pretty wide angle capability.