It depends on what you want to do. If you want to build your own websites or build websites for friends or small businesses, then I would say PHP is your best best. It is free and you can build sites quickly with it. But make sure you understand how to build sites that are secure and hack proof using PHP.

If you are looking for a career path and/or hope to work on large scale web sites/web applications then you are better off getting really good at .NET using C#. You can make more money as a good C#/SQLServer developer than you can as a PHP/MySQL developer. Especially if you want to be a consultant or a contractor.

We just went through a round of interviewing to hire a .NET developer and the rates that some of the really good developers were getting were quite high ($100+/hour). I don't see any PHP people getting those kinds of rates.