I used to own an S3IS but switched to a Panasonic FZ18K since the S5 has a bit of fringe issues in its photographs. Canon might have solved it this year though.
Cursory differences between the two

In any case, you can't go wrong with either one. However, if you aren't entirely sure about what camera to buy, I'd suggest checking out the SX100IS (10X, 8MP) which is definitely a step above the usual P&S cams. The main benefit? You save about $100 compared to the S5 and FZ18.