While I personally use Django/python, going with Rails in quite beneficial as opposed to Scala.

Firstly, there is a huge established community to help you out and you can readily find programmers to hire.

Second, rails (or django) will allow you to get up and running quickly and will let you figure out whether your app is viable or not. If it works out, you can then start thinking about scaling and such. For now, use an established web framework(Django, Rails, Cakephp) and get it done.