I'm creating a PHP comet based application. Which web server must i use within the after sales? Apache / nginx (this is exactly what most forums say) or something like that else could be better?

I would suggest not using PHP whatsoever for this function. PHP is synchronous, therefore every open comet connection will block a PHP process (or Apache process if mod_php). Here an asynchronous language will be used, JavaScript for instance. (See Node.JS. If you work with comet for any chat, take a look at node_chat)