You probably won't see any difference in the quality of pictures coming from either the S90 or SD4000 IS, or even from your current camera. What the two new cameras offer are lenses that are faster, meaning they're be able to shoot in lower light. They also have lenses that are wider, so you can capture broader landscapes, or get more of a group into a shot without having to back up. But the maximum focal lengths are shorter on both cameras too, meaning you won't be able to zoom in quite as close. Your current camera has a 35-140mm lens, the S90 and SD4000 IS have 28-105mm lenses.
The S90 has more manual controls, and it can shoot RAW files, but it can only record standard definition video.
The SD4000 IS lacks total manual control and can't shoot RAW files, but it can shoot 720P HD video.
If you typically shoot in automatic mode, you'd probably be better off with the SD4000 IS, because of the HD video feature.
Im debating btwn these 2 cameras. Which one is better and whats their difference? I've had the Canon Sd 850is so im due for an upgrade and im looking into these two but which one throws better photos and performance?