What is more important - pressure or delivery rate?

The right criteria to define the high-pressure cleaner performances is the washing effectiveness. The elements that influence this value are: delivery rate (water volume processed by the pump in a unity of time) and pressure (l/h water power on a surface). The higher the delivery rate, the faster the washing phase (less rinsing time); the higher the pressure, the deeper the washing (better dirt degreasing).

It is often emphasized the pressure only (also because the pump size and the consequent manufacturing cost are cheaper), but delivery rate is as important as pressure.