This is meant to go along with the excellent
HPA FAQs post. However, here's some info about CO2 as well. When you choose your gas source, you need to consider a few things.
CO2
Carbon dioxide tanks are the most common tanks for beginning players. They're small, cheap, offer plenty of shots per fill, and can be refilled just about anywhere. Unlike HPA tanks that hold highly compressed gas, these are filled with liquid CO2. These tanks are labeled in ounce capacity instead of volume with common sizes being 9, 12, 16, 20, and 24 oz. Since they all store liquid CO2 in the same condition, any refill station can be used on any size of CO2 tank so long as it can connect to the nozzle. CO2 only takes liquid form under highly pressurized and/or cold conditions. Here's a basic chart for liquid CO2 vapor pressure at various temperatures:
*C / *F - ~PSI
0 / 32 - 525
5 / 41 - 590
10 / 50 - 660
15 / 59 - 740
20 / 68 - 820
25 / 77 - 900
30 / 86 - 990
35 / 95 - 1080
40 / 104 - 1175
This liquid evaporates very easily under normal atmospheric conditions which provides the gas pressure to operate a paintball marker. However this evaporation can make CO2 behave very erratically. When dealing with pressurized gasses, any time you have a drop in volume or pressure ( when firing a paintball marker, both happen, ) the remaining gasses must use energy to fill in the void. This loss of energy results in a temperature drop in the gas. The liquid CO2 also cools down as it evaporates just like you cool down when your sweat evaporates. As the liquid CO2 cools down, it becomes harder to evaporate so you get less pressure. Granted, this is most pronounced when rapid firing; people popping off shots here and there won't have this problem as much since the tank can slowly warm up from the air around it. But it makes playing in cold weather difficult since the tank can't warm up much if at all. Also, there is the danger of siphoning liquid CO2 into your marker internals and freezing them up. Most of the time, this is only an annoyance since your marker just stops operating. On rare occasions though it can result in throwing your marker into full auto or even causing damage internally.
HPA
High pressure air tanks ( HPA, also known as nitro or N2, ) are larger and have a higher initial cost. These tanks are designed to hold regular air at extremely high pressures, anywhere from 3000 psi up to 5000 psi. To control the tank output, they have built in mini regulators on the nozzle. Though they usually don't perform as well as a dedicated regulator like a Palmer, they do make the output pressure much more consistent and independent of such variables as temperature. These regulators come in two flavors: high ( around 850 psi ) and low ( about 350 psi or lower ) pressure. Some are even variable. Be careful when buying a tank to make sure you get the right kind. For example, you can't run a stock Tippmann on a low pressure tank. High pressure valves usually have a faster recharge rate so people looking to get high rates of fire will want a high pressure tank. Also most low pressure markers come with their own regulator on the marker so you can regulate the 850 psi output down to the marker's favored operating pressure. Most people just get high pressure tanks since they can be used on any marker.
HPA tanks come in various size and pressure combinations, often abbreviated as 72/30 or 92/45. The first number corresponds to the tank's volume, usually in cubic inches ( CI, ) while the second number refers to the internal pressure the tank can handle. Again, it does not indicate output pressure. Basically, the larger the tank volume and the higher the internal pressure, the more air can be compressed inside giving you more shots from your marker before the tank is depleted. Common volumes include 45-48, 68-72, and 91-92 CI with common pressures being 3000, 4500, and 5000 psi. One thing to remember on tanks is that it doesn't do you much good to spend the extra cash on a 4500 psi or 5000 psi tank if the local store can only refill up to 3000 psi. Any place that can refill HPA should at least have 3000 psi capabilities, but you can't always guarantee 4500 psi and above at all stores, including SCUBA and paintball pro shops. Now they can still refill the tank, but not to its full capacity.
Tank Protection
The material the tank is made of ( steel, aluminum, fiber wrap ) will determine its overall weight and what kind of maintenance needs to be done on it. Steel and aluminum tanks, while heavier, are also sturdier and cheaper than fiber wrap. These tanks can take a serious beating without suffering any real damage. Fiber wrap tanks are made with layers of fiberglass and other lightweight materials. They are much lighter than metal tanks but aren't as durable. It helps to give them some extra protection and almost always are carried in some sort of sleeve or other protective wrap. Say you dive for a bunker and accidentally bounce the tank off a rock on the ground. A metal tank might have some dings on it, but nothing else. It's possible, though not necessarily a given, that a fiber tank could be scored and weakened to the point it's no longer safe to hold the pressure inside. Now, that's a worse case scenario, but the possibility is there. So long as you have a good wrap and don't do anything stupid with it, fiber tanks will survive any game.
Availability
Before you get a tank, first check your marker. Some either don't work on CO2 or strongly recommend against it. Next, be sure to check for refill availability in your area. Check the local paintball stores, fields, SCUBA shops, even welders to see if they can refill your choice of propellant. Check on price and, in the case of HPA, pressure limits. Owning a 5000 psi HPA tank isn't so great if you can only get it filled to 3000 psi. Note that you still can get it filled, just maybe not to the whole capacity. Or maybe you're fine with only partial fills because you at least have the capacity to go higher for when you go to larger events that can fill higher pressures. Take travel distance into account as well. Driving 20 miles each way for a tank refill isn't much help if it's not on your normal route, like on the way to work or to the field.
Size & Cost Effectiveness
Many people ask what size of tank they should get. This is really up to the player since any size tank can be a viable option depending on how it's used. If you don't shoot a whole lot during games, then you may be able to get away with a small, light tank. If you're the suppressive fire specialist, you'll want some more air to work with. Here's a few numbers to work with. My mostly stock A-5 can get almost 600 shots out of a 47/3000 HPA tank. My current 91/4500 tank gives me about 1600 shots. So first, figure how many paintballs you plan to shoot in an outing. Now, what kind of games do you usually play? Quick rec games allow you to re-supply between each match while larger, longer games like scenarios require you to be out in the field for longer periods of time, away from any re-supply stations. Also, what's the refill situation like at the field? Are refills cheap? Free? If so you may be able to use a small tank and have it filled multiple times throughout the day. If not, you may be better off getting one large tank and using that through the whole day. Considering these factors will allow you to avoid getting something that far exceeds or falls short of your needs and will save money.
As for overall cost, CO2 at first appears to be the cheapest route. The tanks can be ten times or more cheaper than an HPA tank. Since they're all metal ( at least I've never seen a fiber wrapped CO2 tank, ) they don't require extra protective sleeves. And because liquid CO2 can evaporate to fill a volume much larger than the air inside a large HPA tank, you will almost always get more shots per fill. However, your firing habits will affect this greatly. As CO2 cools off, it gets denser and requires more of it to fill the same volume as when it's warm. Now even on a day of heavy shooting, a 20 oz tank will likely get more shots than a 68/4500 HPA tank, but the player will determine the margin.
However, as mentioned above, CO2 fluctuates output pressure depending on temperature, both exterior and interior. To help with these problems, you can get anti-siphon tubes in your tank, or install regulators and expansion chambers that control and stabilize the CO2 to make it act more consistently. If this is the route you choose to take, suddenly that cheap $20 CO2 tank turns closer to $80 or more by purchasing other add-ons. Also, if you plan to switch to HPA in the future, those extra purchases will be near worthless.
Wrap Up
Often times, HPA can be much cheaper in the long run compared to CO2. It provides a more consistent pressure than CO2 and performs well in all sorts of conditions. HPA can also be used with any marker with no modification necessary, something CO2 can't do. If you buy one good tank, you may never need another ( unless you decide to upgrade the capacity. ) However, HPA refills aren't available everywhere. Or perhaps you never play in cold weather. If either of those are the case, CO2 may be for you.