ASP.NET is great for data driven web apps but its true, PHP is more marketable a skill to have as a developer. It really depends what kind od work you want to do. To corporate world is still hooked on microsoft technologies while freelance type projects prefer PHP. At any rate, while you are making up your mind, bone up on your SQL and CSS since they are useful for both.
Keep a look out for what happens with microsoft's visual studio 2010 though. It is going to be the first software to incorporate both ASP.NET and PHP scripting in the same development environment. Not that you will be able to combine the languages on a single project, but the interface will be the same.