Comet and Long polling imply writing part of your code in client side and part on server side. You would need in server CometD or Ape to been able to connect to a Comet server, i find that installing them are usually a pain in the ass.
The option that i have found way more easy is to have Node.js with a "comet" module. Faye(also available for ruby) is a long pole pub sub server quite usefull for chats and Socket Io is a great way to have a page updated constantly to sockets(websockets with flash fallback).
Faye Site:http://faye.jcoglan.com/
Socket Io:http://socket.io/
