Here's a review for the FZ18 that might help explain how RAW images can help improve the shots you take.
The hot shoe of the S5 could be an important factor in getting quality images. The FZ18 does not have a hot shoe. Small image sensor digicams struggle in low light to produce quality images. An external flash can make a big difference, if you are willing to pay the money and carry the flash with you. It could cost more than the camera to get a good quality, fairly powerful Canon flash. Most recommend the 430EX model, because it offers good power and good bounce flash capabilities. You might be able to find the older 420EX for a lot less, and still have a very good flash.
Here's a review of the S5 IS from the same site above;