PHP Vs ASP

I've been trying to find an educated comparison of ASP and PHP on the internet, can someone point me in the right direction or would someone care to voice an opinion? We are considering the possibility of streamlining our development of websites which are currently hosted on both NT and UNIX. Currently we tend to use perl on UNIX and ASP on NT, if PHP compares well to both of these we may well decide to develop our sites using PHP in future. Are there any pitfalls in going down the PHP route that anyone can tell me about? I'd be grateful for your opinions or direction to a site that might discuss the above. Thanks Owen.