I bought a brush from gardiners last week with 4 jets, I reckon this allows more water to flow through the hose quicker, making the job quicker as more water on the glass.
I had 2 identical sections of glass, ( I have my pump rigged up to van battery no controller just switch ) the first one I done with the engine turned off and used just over a quarter of 650l tank, the next section I left the van engine running whilst working and used just over half a tank.
On my normal brush heads with 2 jets the pump runs, cuts out, runs etc etc this is just the pressure switch kicking in, with the 4 jets this don't happen as much and the pump has a continuous purring. When the 4 jets are used and the engine was left running on the van the pump is really beefed up and the flow of water on the brush was very nice to say the least.
I think I will fit 4 jets to every brush