As somebody who has administered Mono apps in production, Mono is a huge pain in the arse. This was a year or two ago so mono may have improved, but you're always going to be somewhat behind the cutting edge, and you'll be suffering through much more sysadmin headaches than if you were running a python app. This essentially means you've got to stick with Windows if you want to run C#. Any why would you tie yourself to a dying ecosystem like that?