Howdy, good question! Quality commercial grade pressure washers from brands such as Gerni, Crommelins, Subaru, AR Blue Clean will use brass pump heads which are stronger, can handle more intensive use and have a longer lifetime.
Quality commercial grade pressure washers will also use solid ceramic pistons to avoid corrosion and thus also extending the unit's lifetime.
Domestic pressure washers will typically use an aluminum pump head and solid steel pistons. These are lighter in weight and suit household cleaning requirements. When looking at domestic washers, again its best to opt for a quality brand (as listed above) as they will use non-corrosive aluminum which will prolong the life of the unit, as compared to cheaper brands which are often prone to corrosion.
Thanks!