There is no "comet vs. ajax" dichotomy. Comet employs ajax as a means to interpret data pushed from the server. If your application needs asynchronous, live updates, then you need Comet or Flash. If you use Comet, you'll undoubtedly be using Ajax as well.
Next stupid question(s):
Even though I've written quite of bit of AJAX (Javascript / PHP / MySQL), I have not yet committed to any technology for my release 1.0.
If I wanted to put together a prototype using Comet instead of AJAX, how would you suggest I proceed?
Also, from a business point of view, what are the major pros & cons of comet vs ajax? (hosting, finding help, etc.)