If the OP is interested in having the same programming language on both the browser and the server side, I'd advice to keep an eye open on Ruby Volt Framework http://voltframework.com/
Are you sure? I dont think Sinatra outperforms nodejs, although I could be wrong, this is just a broad perspective of Node's speed and Ruby's speed. Any benchmarks or anything to enlighten me?
Then again, Node comes with the convenience of having the same programming language on the both the front and the back end of things.
If you don't need performance, it's just preference. Stick to Sinatra if you want to bang out the app quickly.