Stipple went with Rails for the sole reason that we were able to iterate and release very quickly in the beginning when I was the only one coding on it, and my expertise is in Ruby and Rails.

We're now heavily Backbone-based on the frontend, however. You should use what you're most comfortable with, as being able to iterate quickly is most important at the start of a project.

As mhd says, the backend is really more important. We're using MySQL, Redis, Cassandra, and a custom Erlang/mnesia based store in ours.

(full disclosure - I'm Stipple's co-founder and CTO)