order by RAND() oppure no ?

[The knight]

Devo mostrare un risultato random da una query, secondo voi è meglio usare order by RAND() oppure fare una doppia query tipo questa:

$count = $wpdb->get_var("SELECT COUNT(*) FROM $wpdb->posts
    LEFT JOIN $wpdb->post2cat ON ($wpdb->posts.ID = $wpdb->post2cat.post_id)
    WHERE $wpdb->post2cat.category_id=6
    AND $wpdb->posts.post_status = 'publish'");

$rand_num = rand(0, $count-1);

$querystr = "
    SELECT $wpdb->posts.*
    FROM $wpdb->posts
    LEFT JOIN $wpdb->post2cat ON ($wpdb->posts.ID = $wpdb->post2cat.post_id)
    WHERE $wpdb->post2cat.category_id=6
    AND $wpdb->posts.post_status = 'publish'
    LIMIT $rand_num, 1