That's a tough question. It depends on a lot of things, but mainly your requirements. What are they?
PHP is open source, which means it's free and so are the servers MySQL and Apache.
ASP.NET is free to use but the server side stuff costs a fortune. Windows Server is a must and a SQL Database is also needed.
ASP.NET also works off the .NET framework, which in many ways makes it easier to build sites as a lot of it is already there and just needs tinkering. It's only when you get down to complex stuff that you have to really use your brain.
PHP is more a manual language, everything is done by hand but PHP does have built in functions to make things like sending an email easy.
I couldn't say any one is better then the other but I would say I get more satisfaction from building sites in PHP.
I could go on for hours but that's a small brief.
