The price difference may be justified by things like better flow rate, higher or lower pressure capacity, adjustability, consistency of output, or specific features like a regulator that can be rebuilt more easily. Flow rate is how well the tank lets gas move into the marker so that high rates of fire don't let marker get starved for gas. Pressure capacity is either 3000psi, 4500psi, or 5000psi. It refers to the maximum internal pressure of the tank. CO2 vessels are around 700-1800psi, because the CO2 turns into a liquid under that kind of pressure. Adjustibility is whether or not you can change the pressure of the gas being fed to your marker on the tank itself. Consistency of output is the exhaust regulator's ability to keep a steady feed pressure from shot to shot. The regulator is the part of the tank that controls how much air pressure is coming out of the tank when it's valve is open.
The easiest transition to nitro for you will probably be a preset output pressure (and therefore less expensive) tank with standard ASA threads. Look for something in the 800 psi neighborhood. Most tanks that are preset are set for this approximate pressure. Some are set to vent 450 psi, or whatever.
Most tanks will not have much difficulty keeping up with the flow rate demands of your marker. Tippman A-5s cycle quickly, but not so quickly that you are likely to have problems here.
A lot of people think that because the nitro tank is larger than CO2 tanks, the capacity should be greater. However, because CO2 makes a phase change (from gas to liquid) under pressure, it stores more efficiently by mass. To get the number of shot from a nitro tank that you would get from a 20 oz CO2 tank, you will need a pretty big nitro tank, rated at a high pressure. The main benefit from nitro tanks is that controlling velocity consistency gets much easier when the gas you use doesn't change phase as it changes pressure. One trades off some shot capacity for the ability to have less fluctuation in chrony speed. I suggest a 68ci tank with 4500 psi capacity. This is middle of the road, and it will be acceptable for most people. A-5s are not gas hogs, to my knowledge, and the 68ci size will keep you with a reasonable balance between weight and volume.
You don't need a regulator (other than the one that the tank has installed in it), but to take full advantage of the better cosistency in energy storage available with nitro, you should plan on getting one eventually. This is a great place to spend a lot on higher performance, because a regulaor with ASA threads will work with almost every marker made. Having well regulated pressure means that shots arc about the same from shot to shot, making it easier to aim at distances and walk balls up to an opponent.
Long and wordy, but I hope it helps.
