You can use the function update_user_meta
for logged in users and the php function setcookie
for the visitors. Here is a start how to work with update_user_meta
You can run this function on submission of the form or by running it by AJAX, i would recommend you to run it with AJAX.
function wpse_update_user_commentform() {
/**
* Get values from the selected
* Dropdown as tha variable $side
* Change this to your dropdown name
*/
$side = $_POST['dropdown'];
$comment_id = get_comment_ID();
// Update the comment meta "comment_side" to the selected value
update_comment_meta( $comment_id, 'comment_side', $side );
}