I would also heartily recommend C# and .NET over PHP.
My own personal bias is that C# is a vastly better language than PHP but I am sure you have enough experience in each to have your own opinion.
ASP.NET MVC is more maintainable than plain ASP.NET WebForms and it is a lot easier to implement TDD (Test Driven Development) in MVC which (again my opinion) leads to more maintainable and higher quality software. Depending on your experience however, it could be a lot faster to get something up and running in WebForms. You can mix and match to some extent though. It seems common these days to design the main parts of an ASP.NET site in MVC and to build the administration screens in WebForms.
You could also use System.Web.Routing in WebForms at first and migrate over to MVC over time although again this might be difficult depending on your experience.
One thing, you do not have to deploy on Microsoft Windows. The Mono project supports ASP.NET projects on Linux very well. If someone tells you otherwise, I doubt they have tried it. If you go this route, you could also consider an Open Source database like MySQL.
MySQL can handle very large web loads with high reliability. Again, I doubt the naysayers have much direct experience with it. There are other DB options on Linux of course.
Deploy on Windows if you want. I just wanted to say that the .NET/PHP decision does not limit your OS platform decision as some suggest.