You don't choose a language in such case as you don't build with Ruby nor with Python. You build with the language and a web framework such rails or django.

There are many things to consider. And most of the issues ar non-technical

- will there be support for the frameworks, language etc long enough?

- are the coders competent with the language / framework

- what limitations exists on deployment

- what limitations the customer imposes

Of course if there's are no non-technical issues to consider, go for the technically "best" solution.