Chat application AJAX polling

What would you recommend?

XMPP through BOSH

There's no need to invent your own message format and transport protocol when somebody else has. If you try, it'll slowly grow to be just as complex as BOSH but without the benefit of third-party library support or standardization.

You might also want to look into Comet.

It's used by GTalk, Meebo, and many other chat applications. A few years ago when I was experimenting with it, there weren't very many libraries or details about server architecture to implement it, but it looks like there is a lot more stuff out now.

Have a look at the cometd project for more technical information.