S230=small and portable
S30=bigger, but more capable.
If you don't need an ultra-compact, get the S30.
If size is an important factor, or you aren't interested in manual controls, get the 230. I have one because I wanted a camera to take places I normally wouldn't bother taking a camera. Good shots. To muddy the waters more check out the S400.
If you want something as portable as the S230, but weather resistant, look at the Olympus Stylus 300. It has a 3x zoom too. The only reason I didn't buy it was becase after looking at pictures on this site I thought the 230 pictures were better.