To start off AS3 is harder if your an artist because things such as buttons now have double the amount of code needed to do the same thing in AS2. But from a programmers point of view its easier to relate to in how its utilized and how the extra code implemented gives more control and organization to a large project.
I could ramble on or just give you this link to a site that has Identical game tutorials in both AS2 and AS3 and let you deside for yourself which one is a best fit.
AS2 and AS3 tutorials
p.s. this is link takes you strate to the AS2 tower defence tut. you view the list of other tuts. just click the tutorial tab on the orange bar. Hope this helps so far its the best site I know for begining AS2 and AS3.