In general, increasing the jet size will lower pressure while decreasing the opening on the jet will increase pressure. Rather similar to putting your thumb partially over a garden hose while spraying. Of course, the performance characteristics of the pump also come into play and limit how high the pressure or GPM can go.
Both pressure and flow rate are important. I tend to favor higher flow rate over PSI. But that can be debated. The carpet construction and types of soil can alter what works best from one job to the next. Deeper carpet pile might respond better to more pressure.
Higher flow rate can provide more thermal energy (heat, but not necessarily higher water temperature) to the fibers which helps especially with oily soils. Higher flow rates also help flush out particle soil which comprises a lot of the dirt in a carpet.
Your vacuum needs to be able to keep up with the flow rate you use or you just get the dirt wet without removing most of it.
