One of the other issues to consider is how large each development community is. While I have limited experience with ASP of any flavor, the one advantage I have found with PHP is the amount of free support in some of the most simple areas - such free available functions that developers share or code ideas. It also appears that there are many more framweorks in the PHP world such as Code Igniter, Cake PHP, etc; not to mention language built on top such as Ruby. The other thing to consider is that with PHP, you can put that on any server... whereas Microsoft is locked to a proprietary server (which could cost a lot of money over time).
Just some thoughts,