You need a high flow rate to get a good fan. So it uses more. Also less accurate if you need to avoid the top of a frame or something, but they have their uses.
Fan jets don't use more water, you just have the brush on the glass for less time.
Every window regardless of the amount of dirt on it needs a certain amount of water to get a good finish. Now you have two choices. First choice less water, which results in longer on the glass. Second choice, more water shorter time on the glass.
A fan jet with the pump on full power shouldn't use any more water than a pencil jet with the pump on full power, but for a fan jet to be effective the power on the pump has to be turned up. Not a problem I should think because you simply can work faster.
Now it is a different storey for people who want to lift the brush off the glass to rinse separately, they may or may not use more or less water with a fan or pencil jet, this is not covered in the above as in my opinion it is a complete and utter wast of time, and water.
Peter