The strong point of the S90 is better performance in low light (without flash).
Most small camera's photos will have too much noise when the ISO setting goes above 200 or 400.
The S90 will have too much noise when the ISO setting goes above 1600.
That is equivalent to a 2 or 3 f/stop improvement.
Not outstanding, but an improvement.
And the S90 does have manual controls for those who want them.
Is the S90 that much better than the SD940 to justify the price? I'm mostly looking to take pics of my kids and won't do much manual.