C# is a powerful language that is highly expressive, has a great type system, excellent compiler and great IDE, excellent async support, fast HTTP stack in standard library, blessed application development framework and object relational mapper, etc. Basically everything you could ask for in an ecosystem for developing a long-lived web application.
Python is a scripting language which is great for one-offs and has great data-science libraries but lacks static typing, poor IDE experience, slow runtime performance, poor portability because of it's reliance on FFI, poor long-term application maintenance track record, no blessed frameworks, competing and incompatible async solutions, etc.
Tbh I don't think there is much contest. Yes you can write a bunch of Python quickly that will do something but if you already know C# which is much better suited for the task it's clearly a better choice.
