For the photography gurus: how much difference will the larger aperture make (f/1.8 vs. f/2.0)? Is it worth the extra money?
I am not a guru, but IMHO, in this particular case, no.
Also, the S110 is significantly smaller in everyday use. If you're going for a compact camera, that probably matters. An S110 will easily fit in your pocket. A G15 won't, or, at least, not that easily, and not always.
One real practical advantage of the G15 is the swivel screen. That I loved, and I still miss even today when I use my dslr.