As you mentioned you are creating a web-based JavaScript application that regularly sends information to, or retrieves updates from, a server then in my opinion you should use WebSocket(especially you are developing in HTML5), which reduce the amount of bandwidth your application uses.
In term of performance, I would chose WebSocket aswell, by doing some measurement experiments e.g averaging a round trip time for 100 requests at a time, WebSocket has a lower round trip time. Here is a link of a performance test might tell the result: http://www.peterbe.com/plog/are-websockets-faster-than-ajax