Originally Posted by Mr. Sakamoto
Would it matter if I get CO2 or HPA?
Yes. HPA systems do not have the pressure fluctuations caused by evaporating CO2 vapor, so they tend to be more consistent shot to shot than CO2 tanks, resulting in slightly better accuracy. This is much more important in fast electronic guns than in a pump like the Trracer, so the difference would be very slight.
As an aside, most nicer electronic guns should not be run on CO2 at all, as if liquid CO2 gets into their solenoid valves the cold temperatures and expanding gas can damage their small O-rings.
The other big advantage of HPA systems is that they are much easier to fill than CO2 tanks. Filling a CO2 tank properly requires a trained employee of the field to take the time to carefully fill your tank himself slowly, hopefully using a scale. HPA fill stations on the other hand are much simpler to use, often with just one hose to connect and one button to push. Because of this, most fields that offer HPA and CO2 fills will include HPA fills with the cost of entry and make the fill stations self-service while CO2 fills are charged per-fill and require you to get an attendant to do it for you. Therefore, HPA is a much more convenient gas source for most players.
On the other hand, if you're going to be playing on private property, or on fields that do not offer HPA fills, this point is moot.
CO2 tanks offer two advantages:
First, CO2 tanks are much less expensive to purchase than HPA tanks. This isn't much of an advantage on a field that offers unlimited free HPA though, as in that case the HPA tank will pay for itself after a number of fills.
Second, CO2 tanks are usually lighter and always more compact than comparable HPA tanks, so they make for a smaller setup.