G15 is f/1.8-f/2.8, the S110 is f/2-f/5.9. That right there is reason enough, but you can add on an external flash, have more intuitive button layout, and more. I'd easily spend the extra $50 for a G15 over a S110.
+1 but you do loose out on 24mm to 28 on the wide end but I actually prefer the bigger body and the ability to ad a flash. Especially if you have a 270 or one of the smaller Canon flashes.