Many manufacturers talk about the horsepower of their pumps. Horsepower can be measured using a "service factor" or a "true rating" system. The first describes the initial thrust as the pump first engages, but immediately the pump output falls to a lower, continuous horsepower level. When a spa manufacturer claims they have a 6-hp pump, they are describing the "initial thrust," not the horsepower the pump delivers as it runs.
Efficient plumbing, better jets and flow control are more important than horsepower. A lower horsepower pump in a more efficient spa will produce just as much jet power as a higher horsepower pump. And remember, you are looking for a comfortable but strong massage, and not looking for acceleration up a mountain road or a punishing hot water experience!