The G9 can use the INON Wide Angle 67mm lens to get a decent WA. I do not believe that it can be used by the G10 (in Canon or Ikelite housing) because it is a 28 vs 35 mm lens, so way too much vignetting to be useful. I understand that the WA choices with the G10 are available, but are they actually better?
G10 has the same size sensor as the G9 and tries to squeeze more resolution out of it. I am not convinced it works. In general you want more sensor area per pixel, not less. The G10 goes the wrong direction compared to the G9.
They are both awesome cameras. I decided to go with G9 instead of G10 because I had the INON WA lens already. At first I was intrigued by the 28mm wide angle, but when you try to get WA under water, the story changes.
From what I have seen, except for video, they take the same quality pictures.