Saving value of a selection option in comment form as comment meta

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 );

}