I like airsoft too, I play with my cousins quite a bit just around our yards.
I would suggest either getting a really good electric, or a decent spring. Springs you have to cock after each shot so they can't be auto or semi-auto, but they are a lot cheaper, and you can get a decent one (300FPS) for only around $30. Electrics are more expensive because they can be auto or semi auto, but unless you buy an expensive one, they won't go near as fast as springs. You could get an electric that goes about half the speed of a spring for about the same price, and you have to recharge the batteries. Never tried gas, but I think they are more expensive than electric.
That's just from my experience with them, so I may be wrong, and you may be able to look around enough to find a good deal. It depends on how much you're willing to spend. You can spend a lot and get a good auto/semi-auto electric, or spend a little and get a good spring.