In summary:
CO2:
Cheap, lighter, more effecient, less maintenence, easier to use.
HPA:
Less Shootdown. Not effected by cold weather. Better FPS stability. Does not harm pneumatics.
In general, CO2 is usually the best propellant for newbies to use. HPA won't provide them any advantage since they won't fire the gun enough to have shootdown, or notice the FPS problems.
But a PRO will notice those problems. If you are going to shoot more then 200 rounds a game, you will notice the major problems with CO2. Your tank will freeze, your FPS will spike and cause penalties, and it might damage your pneumatics.
These are serious issues, and its worth the cost/weight/effeciency issues for the other benefits.
Co2 is stored as a liquid. Much like propane, and its measured by weight, NOT pressure.
HPA/N2 is stored as a gas. A 68/3K HPA tank is 68 cubic inches, and the gas is 3,000psi.
Its roughly equivalent to a 45/45 tank (45 cubic inches, at 4,500psi).
Its usually best to avoid the 45/3K tanks. They are very cheap, but hardly supply enough gas for even one game.
What if you don't like CO2, but your local field does not have HPA? There are options.
For electros, you can use a mixture of anti-siphons, regs, expansion chamers, remotes, and drops to get pretty good results.
For blowbacks, you can use a siphon tank.
Nick