Here's a shortcut, pick the one you and your team knows well enough to get the job done as fast and as reliably as possible. Programmer productivity beats technical sophistication every time.
Also, if you're worried about "scaling", know that almost all web app scaling issues are related to caching strategy, database queries, database structure, and using a queue where appropriate. Rails vs Django vs node vs PHP Framework X vs Java/Scala vs .NET won't change those problems for you.
Choosing your instruments is as much of an engineering problem as the problem itself. Whatever you wind up with, make sure you arrived there based on some reasonable criteria. You'll have some give-and-take:
* Is my team productive with it?
* Do the components have an acceptable level of integrity and maturity?
* Is it fashionable enough, or at least not too unfashionable, so that it does not become the deciding factor for anyone who might become involved?
Also, if you're worried about "scaling", know that almost all web app scaling issues are related to caching strategy, database queries, database structure, and using a queue where appropriate. Rails vs Django vs node vs PHP Framework X vs Java/Scala vs .NET won't change those problems for you.