I would argue it depends on the playing style.
If your just a rec-ball player, who only plays once in a while, and doesnt shoot much paint.....
Then HPA would be a complete waste of money. He would never notice any performance benefit, and instead would only notice its heavier, less effecient, and requires more maintenence.
The opposite is true for a typical tournament player. They shoot paint in quantities enough to notice the problems with CO2, and calculate the benefits of HPA to be greater then CO2.
SO, the trick is just knowing where your playing ability is. If you expect to be playing more in the future, then HPA is a good invesment. If you don't, then stick with CO2.
I would argue the single best improvement is a decent motorized hopper. Without one, I don't see the point of getting HPA, unless you play in severe cole