I would pick PHP over .NET everytime... and I'm not a PHP dev!
the main reason is the bloat in .NET - its good at many things, but web serving is always been a "look at the cool kids and try to replicate what they do". So you either have to support IIS-based webservers which I have always found rather clunky (and if you ever go back to one that was written a while ago, getting it working again is always been trouble).
For webservices, I found that WCF is crud. Its nice to code up a webservice that doesn't require IIS, and it all seems nice. But then I saw how PHP could do a web service in 6 lines of code compared to the 6 files of code WCF required I knew I was using the wrong tool for the job (especially when you factor in WCF's dreadful performance).
The last aspect is security - having worked in very high secure systems, one additional thing you can do to help add a layer of security is not to use a monoculture - so a web server written in PHP that talks to a Windows server running .NET is more secure than both running the same OS (ie a zero-day exploit will be used on both servers, but if one is Linux and the other Windows one server is going to be immune).
I'm all for .NET, its a good tool and I use it daily. But experience has taught me to advocate the right tool, regardless of what it is. I hate and loath the "it is Microsoft therefore we use it" attitude as I've seen so many useless systems written with the wrong, but Microsoft, tool. (I guess the same applies to Oracle, IBM and every other vendor or language - the same hate goes for "100% pure java" that was just a way of saying "only buy my shit")
PS. Apache has supported threads since 2.0 came out, it only spawns processes where some 'cgi-style' processor requires them. Also, on Unixy systems spawning processes is as cheap as spawning threads on Windows.(if you want a truly lightweight thread on Windows, google for Windows Fibres to see what I mean). But again, my point is - you don't know enough about what you're criticising and so are making poor choices based on hype and bullshit.