meteor.js is really cool, but it is not ready for production, so it depends on how important your app is. Scaling it beyond a single server is actually not that simple.
Another factor to look at is how extensive the realtime functionality will be. If you just want to make a realtime dashboard for your app, using ajax with a partial to redraw a div every second (or do nothing if the ajax returns no change) is really simple compared to having to add the complexity of another service in a different language (remember that node.js will run on a different port and as a separate process from rails).
If your app is highly dependent on realtime, and you're not planning on launching to a wide audience in 6 months, sure, rewrite in meteor.
Another factor to look at is how extensive the realtime functionality will be. If you just want to make a realtime dashboard for your app, using ajax with a partial to redraw a div every second (or do nothing if the ajax returns no change) is really simple compared to having to add the complexity of another service in a different language (remember that node.js will run on a different port and as a separate process from rails).
If your app is highly dependent on realtime, and you're not planning on launching to a wide audience in 6 months, sure, rewrite in meteor.