Joe Stagner wrote a comparision study on PHP and ASP.NET on Ubundu 9 and Windows server 2008. The results are here. It seems that ASP.NET on IIS7 is better than PHP on apache 2.

From Joe Stagner

I’ve always thought that if high end performance options were part of your needs requirements,  then .NET programming has some advanced options “out of the box” like multi-threading, asynchronous requests, and a number of caching options.

NOTE – I’m not saying “ASP.NET is Faster so you shouldn't choose PHP !!!!  I’ve always contended that the affable simplicity of PHP had some drawbacks for certain advanced applications. (Just as the early learning complexity of ASP.NET can have it’s drawbacks. )

Read more here