It's not really the same thing. Since Derby is natively rendering server side, it's blazingly fast. You get the best of both worlds, render the initial page load on the server, and then all subsequent loads are on the client (with the data being sent over the wire using websockets or browser channel). This means you're getting initial page loads in the 70-300 ms range whereas rendering via PhantomJS and then serving to the client is much slower and much more CPU intensive.

As far as I know, Meteor doesn't even serve those PhantomJS rendered pages to the average user - only to crawlers - presumably for the reasons above.