CO2
As an alterantive, get a 20oz, and (if you don't have one) an ASA (to screw your tank in and feed your front reg). The kit shouldn't cost too much, and then you can just switch between.
HPA is all very well, but wouldn't you empty your tank throughout a days play?
Richard
P.s. I painted my avatar (which I shall probably change in a bit, I have loads)