It's safe to say I agree with almost everything Mint said. After I learned AS3, I wondered why in the hell I was using AS2 this whole time, and i hear it's a very common thought.
You want speed? AS3 only imports what it needs, and has classes like Sprite and BitmapData.
You want power? You can execute code between application startup and runtime. More proof: I'm building a 3D multiplayer game, and it runs smoothly at 60FPS, plus uses socket connections and Adobe AIR.
AS2 is ancient and creates many more problems than it solves. You're better off learning AS3 now, because you'll make the switch at some point. It's a guarantee.
There's a beginner's tutorial in my sig.