[CPops]

I'm not suggesting you use Drupal without a good reason, but if you're worrying about hypothetical scaling concerns before you've ever even touched PHP or Rails in your life, you're probably focusing on the wrong thing.

Both Rails/PHP are solid choices. If your app fits well within Rails' set of defaults and constraints, you should get to market quicker. PHP has its own set of advantages, such as the ease of procuring cheap hosting and the vast number of trained PHP developers out there that can help build/maintain anything you build.