Let's compare and contrast these methods (and maybe others I haven't listed) and talk about the pros and cons of each - right now, ExternalInterface seems like the way to go in terms of flexibility, but is it right for all situations? Are there concrete benefits in terms of execution speed or anything like that? I'm curious - what do we think?
ExternalInferface was created to make communication between JS and Flash easier, so it doens't really make sense to use anything else. Common practice is to check if its available first by evaluating the value of the ExternalInterface.available property before making a call to some JS. This property tells you if the SWF in which you want to call some JS from is inside a container that offers an external interface. In otherwords, if using ExternalInterface will work. If its not available then just use flash.net.sendToUrl. Never use fscommand() as it uses VBScript and can cause conflicts with other VBScript on a page. Additionally, you can only send one argument string with fscommand and have to split it on the JS side.