What is the best method for creating your own Wordpress loops?

I've used both WP_Query and get_posts. On one of my sidebar templates, I use the following loop to display posts from a particular category by using custom fields with a key of 'category_to_load' which contains the category slug or category name. The real difference comes in the implementation of either method.

The get_posts method looks like so in some of my templates:

<?php    
    global $post;
    $blog_posts = get_posts( $q_string );
    foreach( $blog_posts as $post ) : 
    setup_postdata( $post );
?>
    <div class="blog_post">
        <div class="title">
            <h2>
                <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
            </h2>
            <span class="date"><?php the_time( 'F j, Y' ); ?> by <?php the_author(); ?></span>
        </div>
        <?php the_excerpt(); ?>
    </div>
    <?php endforeach; 
?> 

Where the WP_Query implementation looks like this:

$blog_posts = new WP_Query( 'showposts=15' );

while ( $blog_posts->have_posts() ) : $blog_posts->the_post(); ?>

    <div <?php post_class() ?> id="post-<?php the_ID(); ?>" class="blog_post">
        <div class="title">
            <h2>
                <a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a>
            </h2>
            <span class="date"><?php the_time( 'F jS, Y' ) ?> <!-- by <?php the_author() ?> --></span>
        </div>
        <div class="entry">
            <?php the_content(); ?>
        </div>
        <p class="postmetadata"><?php the_tags( 'Tags: ', ', ', '<br />' ); ?> Posted in <?php the_category(', ') ?> | <?php edit_post_link('Edit', '', ' | '); ?>  <?php comments_popup_link('No Comments &#187;', '1 Comment &#187;', '% Comments &#187;'); ?></p>
    </div>

<?php endwhile; ?>

The main difference is that you don't have to reset the global $post variable and you also don't have to set up the post data by calling setup_postdata($post) on each post object when you use WP_query. You can also use the lovely have_posts() function on the WP_Query function, which is not available using get_posts().

You shouldn't use the query_posts() function unless you really mean to because it modifies the main loop for the page. See the docs. So if you're building a special page to display your blog on, then calling query_posts may mess up the page's loop, so you should use WP_Query.

That's just my two cents. My ultimate suggestion, your first choice should be WP_Query.

-Chris

From the WP docs for get_posts:

get_posts() can also take the parameters that query_posts() can since both functions now use the same database query code internally.

The only difference between the two functions is that get_posts returns an array with post records, while query_posts stores the records in the query object for retrieval by the template functions (has_posts, the_post, etc).

They both use the WP_Query object to execute the query.

Creating a second loop is covered in the Wordpress docs. There are some links there for other examples of multiple loops. You'll notice that everyone does it differently, but they all seem happy with their results.