I would go for option 2. Setup a second site using the same database. You then would copy all of the files in the root of the site to the new site. Next step is going to be to determine areas of the template that you want to put 'high-bandwidth' content in, and areas for 'low-bandwidth' content. Let's say that you are publishing 5 modules to the 'right' position currently. A menu might be one such module that could be common to both the low bandwidth and high bandwith versions. A big movie would be high-bandwidth', an image would (should) be 'low-bandwith. So in your template, do something like this for the high-bandwidth version of the site:
<div id="right">
<?php
if(mosCountModules('right-common')) {
mosLoadModules('right-common',-2);
}
if(mosCountModules('right-high')) {
mosLoadModules('right-high',-2);
}
?>
</div>
and then in the low bandwith version of the site:
<div id="right">
<?php
if(mosCountModules('right-common')) {
mosLoadModules('right-common',-2);
}
if(mosCountModules('right-low')) {
mosLoadModules('right-low',-2);
}
?>
</div>
now you need to go into site->template manager->module positions in the Admin of the site and setup these module positions, and then into the module manager and move the modules to their new positions.
You can have two different Joomla! installs running in parallel directories, eg.:
www.yoursite.com/high
www.yoursite.com/low
you just need to specify in the individual configuration.php files.
Obviously you need some kind of 'switch' mechanism so that the users can choose the version of the site that they want to see. Simplest solution is a landing page, but there is no reason that you couldn't craft a button that would allow a user to switch in context without loosing their place whilst in the site... (I'm getting a bit carried away now...) You can ofcourse use browser detection to make the choice for them.
www.yoursite.com/index.php
is not going to be served by either Joomla! install, so it is up to you how you handle this.
As long as you keep your content fairly clean, and use the modules to insert the content for high or low bandwidth users, it will work for you.