In a recent post to the phpslices.com blog there's a neutral comparison of PHP and ASP.NET based on features and a few of the common pros and cons each language has.

Consider i do not have wide experience programming with ASP.NET though i on the past used the Microsoft .NET framework. This is what i think are PRO and the CONS of both, lets consider ASP.NET 4.0 without MVC and PHP 5.3 without any framework.

Some of the pros of PHP include its low learning curve and that it's multiplatform with cons being things like performance issues and that it's more web-centric than some other languages. ASP.NET's pros include its inherent use of the .NET framework and the tools available for it with cons being that it's not Open Source, you need a Windows license to use it and that fewer web hosting providers support it.