PHP or ASP.NET?
These are different fish that swim on different server operating systems (OS). PHP is usually used on Apache web servers while ASP.NET requires a Windows server.
So the choice to make before even considering the "programming language" for the site is... what type of web server? Apache is most commonly available and hosting is usually the cheapest because the OS licensing is free. Windows OS requires licensing and will cost more to host.
The performance difference between these OS is not comparable except for variations in machine resources. Most established hosts cater for both because there is a difference in what each can do when it comes to application support.
When it comes to programming language it does depend on your skills and comparing "PHP with ASP.NET" is not a good choice for comparison because ASP.NET is actually very limiting as it is not pure code and requires licensed software to compile. Then once it is compiled those resources (DLLs) are not easy to update without recompiling.
So a better comparison would be what is the difference between PHP and Classic ASP as they are both pure code. PHP is used on Apache and ASP runs on Windows servers.
PHP coders will argue that PHP is more powerful and you can do a lot more. But I can tell you that there is nothing that can be done using PHP that cannot be done using ASP that is of any consequence when taking into account the fact that Windows applications cannot run on an Apache server.
By "Windows application" I mean all software that runs on your local Windows workstation can be run on a Windows server and interfaced with your web site. For example you can install Microsoft Office products or even OpenOffice on the server and have users upload all sorts of file types for conversion to almost any other type of file for inclusion or display on your web pages.
And this is why I use Windows with ASP in all of my complex web applications. But if you are simply building web sites to run ads and blogs then I recommend PHP on Apache because it is cheaper hosting and add-ons are usually free and abundant.