I've been evaluating these three frameworks yesterday and while I have to say that the speed of Express is really going to be critical in the long run, I think it's premature to use it at this point in time. What I need to do right now is get a prototype up and running quickly so as to keep myself motivated. Sinatra is really the same as Express (albeit a little slower and less of the asynchronous goodness) so I'll have to avoid using it at this time.

Since I'm already familiar with Rails, I'm going to use that as my prototype's springboard so that I can focus more on the client side where most of the action is going to happen. Later on, I'll just replace the backend with Express or Sinatra, whichever makes more sense.

No code samples for now. Gotta keep the suspense going! :-)