Actually the htaccess will probably put a greater strain on a busy server because every js, css, gif, jpg etc file will be processed by the htaccess file where as the plugin will probably be a lot more targeted.
If you do go down the htaccess route I would leave the sub-domains up and empty apart from your htaccess but you could put everything on the main site and delete the sub-domains (as per AWB's suggestion above
).
If you have a dedicated box or a VPS (ie you're not on shared) you have other options such as setiing up redirects in Apache, also I have an idea I did something similar via CPanel and it might be worth looking there too.