Let say a few web agencies are PHP shop for rolling out Wordpress, Woocommerce, Drupal or Magento, CodeIgnite or Yii. Clients are told that PHP-NG will improve the overall performance and responsiveness.
How do Meteor Devshop advertises enough benefits to secure your proposal over PHP?
We have our standard Meteor pitch here: http://differential.com/meteor
But another powerful exercise we’ve started doing is just directing them to our website (http://differential.com) and telling them to click on the nav and see how fast the pages load. They’re usually blown away. Now, we know why they load so fast (they’re already loaded!), but compared to most Wordpress sites, it’s like magic 
I think Meteor has a lot of advantages of PHP. For one, customizing it is easier (in my opinion) than customizing a CMS like Wordpress.
Also, a very big factor you can convince your clients with is the real-time aspect of it. It is real-time by default and if you try to make a PHP-based app real-time, you will have to use a different front end framework to do so. And there is JS on both the server and the client. You only need to know one language and you can do full stack development whereas you would have to find people specifically for front end and others specifically for back end if you did PHP + JS.
Perhaps, a client may need an advice to migrate MySQL data from Wordpress or others into MongoDB, a utility or converter will be interesting.