AS3 is a fully object oriented strictly type language much like java, but is based on ECMAScript similar to javascript.
So syntactically it will look similar to other "scripting" and it can access the same flash player classes that previous versions of Actionscript used, however I would say they are NOT the same. The very fundamentals of the language have changed so much that current versions of the flash player have two engines one for AS3 and one for legacy versions.
That said you can learn whatever you like and both versions will fun in the flash player, but I would go with AS3 as that is a full robust language and is the future of the flash development platform.
These articles are dated but they give a solid look at the syntactic differences in the languages as of AS2
Lingo and AS - One, Lingo and AS - Two