Both are free, so the question is more like... What path do you see yourself going? Microsoft-products route, like IIS, Silverlight, .NET development, and so on, or do you see more of the Apache, PHP, MySQL, Linux, and so on?
Generally the two don't mix, but it *is* possible to run PHP on IIS and so on.
Also, writing raw code in PHP is becoming less and less viable, or at least, needed, as more people are going to frameworks, like CakePHP, or CMS like Drupal, and so on. It wouldn't hurt to learn PHP, but you should learn the basics of SEVERAL different webdev languages. Once you learn one, the others are quite similar.