the biggest reason for the cost difference between the s95 and the likes of the sd4000 is the physical size of the sensor. the s95 uses a 10mp 1/1.7" sensor versus the smaller 10mp 1/2.3" sensor of the sd4000. also contributing to the cost difference is some features such as the control wheel and the ability to shoot in RAW.
for you the bigger sensor would probably be the main reason to look at the s95, it will do better at high ISO for low light/no flash situations. particularly at iso 800/1600 where it is quite clean for a compact camera.
however, the sd4000 does quite good up to 800 (better than most with this sensor size), and at smaller print sizes and web host size, even 1600 could work.
so it just boils down to how much will you shoot low light without flash and whether your print/display sizes will warrant the price difference.