Although airsoft and paintball seem very similar on the surface, there are actually many differences between the two. Because of the difference in how the guns are used and how they operate, these differences include the areas of equipment, cost to play, types of ammunition, maneuverability, and game dynamics.
In both airsoft and paintball, the crucial piece of equipment is obviously the gun, but there are differences in the types of accessories needed as well. In paintball, all guns use a large hopper to load the ammunition into the gun, while airsoft guns use magazines, just like real guns. The advantage of using magazines is that they are easier to store & carry, and it is easier to reload when you use a magazine. In paintball the hopper is quite bulky, and if the gun is not held upright you can run into problems with the paintballs not feeding properly. Also, when you run out of paintballs you have to pour more paintballs into the hopper, which can take time. In airsoft all you have to do is switch magazines.
If you are conscious of your budget, airsoft is definitely the way to go. The guns are generally less expensive and there are fewer maintenance costs. If you purchase a spring or battery powered airsoft gun there is no need to spend money in the future to power your gun, as most batteries are rechargeable and spring guns are cocked manually. Paintball guns use CO2, and when you run out of CO2 you will need to purchase more if you want to play again. The same is true if you buy a green gas or CO2 powered airsoft gun. In airsoft, ammunition is also much cheaper than in paintball. For 2,000 rounds of paintballs you will spend roughly $40 (source: walmart.com) while you can get 5,000 high quality Elite Force 0.20g airsoft BBs for only $10.95. When you do the math, 1 paintball is almost 11 times more expensive than 1 airsoft BB, while they both essentially perform the same function.
The advantage of airsoft ammunition (besides low cost) is the fact that it is small and can fit into regular sized magazines easily. In a standard M16 style magazine you can fit as many as 450 rounds! Paintballs are larger, so they require a large hopper and special equipment to carry extra ammo into battle. The advatage that paintballs have is the fact that when you hit an opponent you have proof because the paintballs mark their targets with colorful paint when they hit.
Because they use smaller ammo, airsoft guns are also easier to maneuver with. Real guns are made to be effective in the field of battle; in a real firefight your gun is literally your life-line. Since airsoft guns are modeled after real-steel guns, they are also made to be maneuverable and easy to use. There are many styles of airsoft guns available for different scenarios. For close quarters combat, where maneuverability is very important, CQB airsoft guns have the upper hand because they can be made to be just as maneuverable as the CQB submachine guns used by law enforcement and military personnel.
As far as game dynamics go, the main difference is the fact that airsoft relies very heavily on the honor system. Since it is not always possible to see if you hit your opponent, other players have to be honest for the game to work and for everyone to have fun. In paintball, this is not a problem because paintballs mark their targets when they are hit.
In the end, both sports can be a lot of fun and a great challenge. Since the games are similar in most ways, personal preference will play a big role in which is a better fit. If you're having trouble deciding which sport you like better, the best way to figure it out is to get out there and try them both! The most important thing is that you find a pastime that suits you well and which YOU enjoy.