What preloader to put on main stage?

1

I am loading external swf into main movie and external swf also loads some content externally.

so i have 2 ProgressEvents, one in my main movie for loading external swf, and other in my external swf for loading its content:

swfLoader.contentLoaderInfo.addEventListener(ProgressEvent.PROGRESS, onProgress);
function onProgress(e:ProgressEvent):void {
}

so which preloader should i put on main stage?
how can i know what is going to take longer, loading external swf or external swf loading its content?