There are two primary types of competitive airsoft guns: AEG and GBB. Yes, we are purpose ignoring the spring airsoft guns, because the only competitive spring airsoft guns are high-power sniper rifles.
AEG stands for airsoft electronic gun, meaning that it is powered by battery. GBB stands for gas blowback, meaning that the gun is powered by gas. Although there are many gas airsoft guns that does not have the blowback feature, they are still categorized as GBB for easy reference.
In this article, we are going to explore the advantage/disadvantage of a AEG and GBB so that you can make an informed decision on which type is more suited to your playing style.
External upgrades for both types of gun are the same. It's the internal that differs.
With the AEG, there is a gearbox, where a strong spring is pulled to fire the BB's. Most of the upgrade occurs either within the gearbox or around it. That means you'll normally need a punch and a screwdriver to take it apart. You may need wire cutters and soldering iron if you are going to upgrade the electrics
GBB upgrades tend to surround the magazine. Usually the amount of power is controlled by the valve. So upgrades means changing valves on each of the GBB magazines. That means you'll need valve replacement tools.
Both types of airsoft guns requires constant maintenance. Air tightness is important to both types of guns. So you will be working to ensure that the air seal is good throughout the gun. Teflon tape and silicone lubricant should be a mandatory in your toolkit.
The FPS (feet per second) of both types of airsoft guns are about equivalent. However, there is an exception when it comes to pistols.
Because pistols are so small, it's difficult to fit a gearbox into them. Therefore, the smaller gearbox simply cannot provide enough compression to fire the BB's at a very high rate. Therefore, generally, only GBB pistols are competitive to rifles when it comes to FPS. Airsoft electronic pistol (AEP) are not as competitive, though they are more consistent (see "Weather" below).
Batteries and gas are both effected by temperature. In cold temperature, battery doesn't last as long as in hot temperature. In cold temperature, gas isn't as powerful.
Based on these physics constraint, the AEG shoots more consistently in different temperature. You just might run out of batteries quicker. That can be remedied with extra batteries, carrying extra batteries close to your body heat, etc.
Cold temperature affects GBB greatly. How fast and how far your GBB shoots depends on the temperature; meaning you'll have very little consistency as temperature changes. CO2 vaporizes at much greater pressure than green or red gas, therefore is less effected by temperature changes. If you live in a region with wide temperature changes, stick to CO2 or even AEG.
Between a AEG and a GBB, the GBB normally have higher recoil. That means the GBB feels more realistic when fired. However, due to the higher recoil, rapid firing a GBB have to be handled with the same care as a real gun. Hence, they make great training weapons. Because AEG has less recoil, rapid firing is easier, therefore, can be more competitive on the battlefield.
GBB rifle magazines are usually real-capacity, meaning that they hold about 30 rounds. AEG, on the other hand, has high-cap, mid-cap, and real-cap magazines. If you don't care about realism, having a 450 round high-cap magazine could make you very competitive on the battlefield (assuming no one else is using tactics). With GBB rifles, you don't have that luxury. You have to carry multiple mags.
The cost of a gas operated airsoft gun is generally higher. Even if the gun prices are the same, the additional real cap GBB magazines will drive up the cost. That is because there are no high-cap magazines for GBB rifles and pistols. GBB magazines cost anywhere from $30 to $50 each.
AEG mags on the other hand are below $10 each. And if you have no problem with the unrealistic high-cap magazine, you may only need one.