What nobody is telling you about going with an HPA tank is......that there is no way you'll be able to play 4 days with it. You may get one day of play using a small(48ci) HPA tank. So you'll have to fill about 4 times more than you would with the 20Oz Co2 tank. I'm guessing by what you've said, I don't know how many shots you shoot per day.
So.....depending on how much you pay to fill either up.....it may actually cost you more to go with an Air tank. Of course air will always work better, despite getting a lot less shots on a full tank.
CO2 is fine for rec ball. HPA is better but has both advantages and disadvantages. In the end the choice is yours.
How many paintballs do you shoot in a day of play? Are you playing at an actual field that includes air fills in their entry fee?
It's all about the angles.