previous_posts_link and next_posts_link, how to return the next/previous post’s title as the url

The functions you are using refer to the next or prev paginated set of posts, not a single post, hence the format they are in. Try using get_adjacent_post() instead.

<?php
    $prev = get_adjacent_post(false, '', true)
    $next = get_adjacent_post(false, '', false)

    //use an if to check if anything was returned and if it has, display a link
    if($prev){
        $url = get_permalink($prev->ID);            
        echo '<a href="' . $url . '" title="' . $prev->post_title . '">Previous Post</a>';
    }

    if($next) {
        $url = get_permalink($next->ID);            
        echo '<a href="' . $url . '" title="' . $next->post_title . '">Next Post</a>';
    } 
?>

You can read more about get_adjacent_posts() here.

This should be noted that the get_adjacent_post() function is intended to be used on a single detail page. If you are using this on a posts page it may behave unexpectedly.