Might be side-stepping your question, but can you use get_template_part?
<?php
get_template_part( 'plans', get_post_meta($post->ID, 'my_custom_field', true ) );
?>
Then create “plans_plana.php” and “plans_planb.php” (assuming that ‘my_custom_field’ can have values of ‘plana’ and ‘planb’).
twentyten and twentyeleven make heavy use of get_template_part if you need some more examples.