It doesn't really matter if you choose PHP or ASP.Net, both can deliver, both have a solid base, both are proven technologies. PHP is a mite cheaper, but ASP.Net is getting there.
Unless you're going to maintain the app for a long time ASP.Net's superior OOP capabilities aren't going to offer you much advantage. But if you've got a non-trival app then the multi-layered ASP.Net approach might be helpful.
Just get the coder you're most comfortable with.
