If you use a tap or trigger you prob won't see any saving in water, BUT you won't have the annoying loop of pole hose to get snagged and knotted, or if you let the tap run on the ground you won't have the keep picking it up. As smudger said you can quickly turn it of to wipe the sills.
I also have the same setup as choppsie, not having a pole hose is much quicker.
I have had the mk 1&2, and the mk2 is a big improvement over the mk1. I know they are pricy, but so is a supalite brush, but they are both excellent products, go and get one.