You can't compare PHP and ROR, PHP is a scripting language as Ruby, and Rails is a framework as CakePHP.
Stated that, I strongly suggest you Rails, because you will have an application strictly organized in MVC pattern, and this is a MUST for your scalability requirement. (Using PHP you had to take care about the project organization on your own).
But for what about scalability, Rails it's not just MVC: For instance, you can start to develop your application with a database, changing it on road without any effort (in the most part of cases), so we can state that a Rails application is (almost) database indipendent because it's ORM (that allow you to avoid database query), you can do a lot of other stuff.
(take a look to this video http://www.youtube.com/watch?v=p5EIrSM8dCA )
