I've recently gone to using only sand. I've done 2 different smokings of ribs, and a smoking of butt just yesterday. Personally, I haven't seen any major pro/con to either method, except for the one major pro using sand - cleanup. I use the WSM water pan filled to about an inch from the top with sand. I have a layer of HD foil on top of the sand, and a second layer that gets replaced with each use.
At this pont, I don't see any need for using water.