As someone who has used both, I have to give this battle to Node.js and Socket.io. The ease of use because of the common client/server code almost blurs the line between the client and server and makes it almost a pleasure to use. Whereas in Play/Scala, there isn't a libary I can use for both the client and server, thus I either have to write my own client library (or use one) and then write my server code. From a maintainability standpoint, Node, in this particular use case, just demolishes Scala/Play.