JavaScript is natively supported in all current browsers with a GUI. To view SWF, you need a plugin.
SWF comes installed or at least has an option to be installed in every browser I know of. (windows only) Javascript in Opera is a separate download. It is a option to install.
You mean Java, not Javascript, I guess. AFAIK Javascript works in the non-java download. At least it worked for me last time I installed a non-Java supporting opera on windows.
If we would choose SWF, this would completely replace the current search box. If we choose JS, this would only be an extension to the current search box. This means users without SWF can't see the search box at all, while users without JS can use the search box, they just don't get instant results.
If it were set up properly, people without SWF would have the normal search box and the others would simply not know about the feature. Unless, they have a browser that automatically downloads and installs the swf components and then displays the content.
The same compatibilty stuff will be implemented with the JS. If people have no JS, they will have no problem. BTW we already have features using JS (remembering your last search string and option for example). People without JS will not see that feature (or any error).
Goba