Not that I care since I'm an OSS developer, but C# is an open standard designed by Microsoft.
So I guess it could be viable, although I would have no idea how to start developing with Mono, but Python comes with my OS distro.
Oh, and connecting to databases and shit most times requires installing gems and libs on your own while most PHP configs are already SQLite and MySQL out of the box.
PHP is much easier to get started with, but FYI Python comes with SQLite as part of the language. If you do want to give Python coding another try, Google App Engine is a very easy way to code and deploy a website, check it out.