My new A-5

well, here's my artical over hpa vs. co2 and i'd say hpa would be your best bet. but hpa's not really cheap.

HPA

You can switch Between HPA and C02. Names for HPA include Nitrogen, N2, and Air. Nitrogen and air are different, but for paintball purposes they do the same job.  You don’t need any modifications to the marker to accept a HPA tank, as long as it is a high output (around 800psi) screw-in tank. Expansion Chambers do not help HPA at all, but don’t harm it either, you can leave you Exp. Chamber on your marker with no difference in performance. Regulators help keep the input pressure of your marker consistent, therefore preventing pressure inconsistency. Regs are not necessary for HPA, but can help out and make your marker more consistent.  HPA works better year round than C02 does because it does not have to change from a liquid to a gas.  You can’t put HPA in C02 cylinders because the cylinders for C02 are not made to withstand that kind of pressure. You can’t put C02 in HPA bottles either because an HPA bottle isn’t made to withstand the cold frigid temperatures of liquid C02.  There are about 3 different pressure tanks you can get for HPA use.  3000 psi, these are normally made out of aluminum, but also come in carbon fiber, you will normally get about 700 shots out of a 68/30 tank (68 being cubic inches, and 30 being 3000 psi as a general rule you get about 10shots per ci).  4500 psi, these are normally made out of carbon fiber, and you will get about 1000 shots out of a 68/45 tank and you get about 15 shots per ci.  5000psi these are somewhat less popular than the others, and are made out of carbon fiber, you will normally get about 1300 shots out of a 68/50 tank and you get about 20 shots per ci.  Before you switch to nitrogen you need to factor in these conditions, how often do you play, what kind of marker do you have, do you play indoor or outdoor, do you play in the winter, do you shoot your marker at high rates of fire.  For example, if you don’t play in winter, and you have a stock tippy, its probley not worth the switch to nitrogen.  But if you have an Ion and you play speedball year round Nitrogen is a must.  Most high-end guns use nitrogen, and say never to use C02 because of problems with the seals and electronics.  You may decide that Nitrogen is the answer, or you might decide to run C02 anti-siphoned and a palmer stabilizer is they way to go.  Some good makers of Nitrogen are Empire Fuel, Macdev, Angel Air(low pressure only) are the more expensive, and some cheaper ones are Crossfire, Pure Energy, Nitroduck.

If you are going to us co2 get a co2
 tank with a ANTI SYPH0N TUBE. This reduces
the amount of liqued co2 and is better for your gun.

Yeah, if you're playing in that cold of weather you kinda have to go with HPA.

I also agree I really don't think I could stand a co2 tank bigger than 20oz.