CO2 is stored in a tank in liquid form. When it is released it quickly converts to gas and the expansion of the gas fires the paintball out of the barrel. Unfortunately, variations in temperature (CO2 going from liquid to gas is endothermic, cooling your gun as you fire), incomplete expansion (liquid CO2 staying in the gun and not completely converting to gas) and different amounts of CO2 being released depending on how full the tank is all contribute to an inconsistent performance of CO2. CO2 can be more consistent when used on a warm day and when an expansion chamber (Compare Prices) is used which helps the CO2 expand more uniformly before firing the paintball.
Compressed Air is not stored in a liquid form so it's expansion is much more consistent than CO2's and it is much less affected by temperature variations. Compressed air tanks, though, must have a good regulator to insure that they release the same pressure and amount of air with each shot. Compressed air tanks come with a built in regulator but most high-end guns come with a second air regulator (it usually is inside the fore grip) to double regulate the air and insure consistent firing. But, if you don't want to purchase a high-end gun, almost any gun can be fitted with a regulator (Compare Prices) to insure each paintball is fired consistently.

