On the frontend Im using Sproutcore.
The question is how the backend is going to look like.
Solutions:
Could CouchApp really replace Node.js on the backend regarding?
CouchApp is nothing more than an easy way to deploy code to your CouchDB. It doesn't expand it's functionality but let's you use what's given easy way. So it's not CouchApp vs Node.js but rather what doesn't CouchApp have from the list of features you need to build your app.
If you need websockets/streaming go with Node.js middleware. CouchDB doesn't have 'em.
And if you give some details about your project I can try to give more complete answer.