Come to think of it, when I was setting my camera up, I was given these choices as well. That must be why sometimes my batteries don't last very long and other times much longer. When I have the IS on, it drains it much more quickly, I guess because it is on all the time. But I've never noticed any hesitation or noises during IS use, or any noises when I depress the shutter half way.
I also have a dial on the camera to turn the IS on or off, so I guess my IS is either on all the time, or off all the time. From what Loupey says, I would prefer it the way it is.