For my next web application, I'm debating whether to use Rails 3.x or Sinatra.
I would like to use the server to provide user authentication, application-triggered emails, a fairly complex data model (behind ActiveRecord), and a JSON data interface with the web client. The client side will use static HTML, static CSS, Javascript/jQuery to render the JSON data into the views. The "policy" for rendering views will be driven by the Javascript code and some of the JSON data. I do not plan to use any dynamic view technologies such as ERB, HAML, or RJS.
Would I be better off with Sinatra or Rails 3.x?
Are there any other questions I should ask before making that decision?