What's the difference? One is looking to see if the variable $_REQUEST['var'] is set or not, the other is looking to see if the variable $HTTP_POST_VARS is something non-null. Quite different things.
Use if(isset($_POST['var'])) - always make sure your variables are set and make sure you know where they're coming from before trying to use them. Basic security, that. You can get PHP to squeal at you when slip up if you want, and in the default distribution it will.
http://www.phpbuilder.com/board/showthread.php?s=&threadid=10226224&highlight=isset
See also [man]empty[/man].