Here are some of my thoughts on the various technologies available. The air-pressure based water weapons are how Larami Ltd. changed the face of water wars. The original style of air-pressure water weapons have water reservoirs which are their pressure chambers and must be unscrewed to fill (e.g.. SS 30, SS50, etc.) The route from the tank to the nozzle is a tube which sits in the water reservoir. Even some of the newer weapons are based on this old technology (e.g. XP 20, XP 40, XP 90, SC 400) The main problem with this technology is that the water-intake tube must be submerged in order for water to be fired. If not, one will be firing only air or mist. This limits the angles at which the weapon can be pointed and still function.
Another style of air-pressure water weapons have the intake tube from the water reservoir to the pressure chamber still based on a tube within a screw-on reservoir (e.g. SS 100). The problem here is that the weapon must be at the right angle when pumping otherwise one will be pumping more air than water. In the end, there is always a little bit of water left in the reservoir which cannot be used. Firing angle is also restricted due to the shape of the pressure chamber. Improper angles will result in more air than water being fired out.
The newer style of air-pressure water weapons employ a separate pressure chamber and a water reservoir which can be filled even when the weapon is pressurized (e.g. XP 70, XP 110, XP 150, etc.). The reservoir style is altered and drains from the bottom instead of through a tube. This allows virtually all water to be used. Also, a slightly greater firing angle can be used thanks to the design of the pressure tanks. However, again, one must be careful how the weapon is held otherwise one can end up firing more air than water, depressurizing the firing chamber very quickly.
The newest technology seen in the water war world is the CPS technology. The most powerful water weapons are based on this (e.g. SC 500, SC 600, CPS 1000, CPS 1500, etc.) Basically, water is pumped into a large, elastic chamber. The water pressure is achieved by the tension on the rubber as opposed to an air-pressurized chamber. Because of this, any angle can be used when firing these weapons. However, as the water reservoir typically have the water intake at their base, the weapon must be level or slightly tipped forward in order to properly fill the firing chamber when water is running low. Air within the firing chamber can result in a sputtering stream.