Well, I use AS2 because I feel it is easier to make games with. With AS2 you can add actions to ANY object. For instance, if you want to make a simple button, you just use the actions panel ON THAT OBJECT and type a simple code:
If you were using AS3, you HAVE to use the actions panel at the top. You can't have any actions in an object.
It all depends on what you feel, I would recomend learning both, it is always best!
You will do yourself a favor to learn AS3, and if you have the energy/drive/need afterwards you can pick up on AS2. Placing code on objects as highlighted in the first response is a bad practice and was discontinued in AS3. Keeping all code on the timeline where it can be easily found makes for better design. You can limit yourself to timeline coding with AS2 as well, but the future will evolve from AS3, not AS2.
The above is correct [inappropriate remarks removed]
"The whole point of Flash, the whole FUN of it for designers was that we could make buttons do stuff, we could make little movie clips misbehave and go crazy"
Dude grow up you can still do all the little tooty fruty stuff you want. AS3 does have a steeper learning curve but it's well worth the trek. To really master Flash you need both sides of the brain.