The best way to do this is by adding ‘user_can_richedit’ filter, like so:
add_filter( 'user_can_richedit', 'patrick_user_can_richedit');
function patrick_user_can_richedit($c) {
global $post_type;
if ('page' == $post_type)
return false;
return $c;
}
Hope it’s useful 😉