Whenever I've tried to reuse the same controller for two URLs, I usually end up with some conditional logic within the controller to handle difference in parameters available from the URL.

I think a better approach would've been for me to subclass one from the other, or have a common parent class, and then handle the parameter differences within each subclass. At that point you'd end up with a 1:1 mapping between controller class and URL anyway.

I don't understand how a monolithic route mapping layer & a separate resource layer is more reusable than a modular resource + route layer. Care to explain?

I find the Sinatra approach leads to less code and less complexity, especially for larger apps. This is because it is easy to extract middleware using Sinatra, since I break up separate resources anyways, instead of the single Application object.