This depends on the kind of work you want to do. If working in very thick metal, get a waterjet pump that is capable of putting a lot of horsepower to the nozzle. Although a smaller pump can machine thick metal, a larger waterjet pump will be faster, and therefore more practical. If cutting mostly thin metal, or doing water-only cutting, get a smaller pump. Smaller pumps are often cheaper, cost less to run, are easier to maintain, more reliable, and are quieter. Bigger pumps, however, allow faster cutting.
Note: The important is horsepower at the nozzle, not the size of the motor turning the pump. It should be take a note, because many pumps have big motors driving inefficient pumps, while other pumps have smaller motors running efficient pumps.
For example, a 50 hp (37 kW) intensifier pump will typically only put 30 hp (22 kW) to the nozzle, while a 30 hp (22 kW) crankshaft pump will put 28 hp (21 kW) to the nozzle. Examine the price and maintenance differences between the two types of pumps, and the lower horsepower pump quickly starts to look like the better choice:
The amount of electricity lost as efficiencies can have a significant effect on operating cost. For example, if electricity costs $0.10 per KWH, and you are wasting 15 KWH per hour, then you are losing $1.50 per hour to heat. In addition, because Intensifiers require additional cooling water, you are losing even more in the cost of cooling water.
article from: http://waterjetapw.com