Major syntax differences between JS and ActionScript?
Well I'm not sure what your career path you want to take design/developing but if you are interested in programming I would suggest you learn an OOP (Object Oriented Programming) language as your first language. Actionscript CS3 is OOP in case your interested.
Once you have the syntax of one language down it is very easy to pickup another language because a lot of flow is that same, such as functions, loops, if/else statements, constants, classes etc. Hope this help you to decide.