I have used both ASP and PHP before. To be perfectly honest, I prefer PHP to ASP. Not for any real reason that I could write a topic on why I prefer PHP over ASP, but more of because from what I have seen there is more PHP out there than ASP. Also, where I have looked for books, etc. to learn the languages there seem to be 10 PHP books for every ASP book, so naturally I chose PHP. Also, my webhost was cheaper to get a Linux server with PHP and MySQL support than is was for a Windows server with ASP... so not a lot of choice there really lol.
I do admit though, the website that I programmed in ASP actually felt easier to do than doing the same in PHP. Whether that is just me or not I don't know?!