I like Python mostly for the great portability and the ease of coding, but I was wondering, what are some of the advantages that C# has over Python?
The reason I ask is that one of my friends runs a private server for an online game (UO), and he offered to make me a dev if I wanted, but the software for the server is all written in C#. I'd love to do this, but I don't really have time to do multiple languages, and I was just after a few more reasons to justify taking C# over Python to myself.
I'm doing this all self-taught as a hobby, btw
There are lots of differences, advantages as well as disadvantages. I guess the main advantages would be along the lines of

Excellent Windows integration, including access to all standard GUI functions and other libraries.
JIT compilation, resulting in better performance than Python, in some or most circumstances. As has been pointed out, this is now also possible in Python.
On Windows, the IDE support is arguably better for C#. Visual Studio is a well established and advanced development environment and the "Express" editions are free for personal use. In a non-Windows environment, it's probably a draw between different editors.

The rest is basically up to personal preference (statically typed versus dynamically, C-like syntax or not, etc.).
what are the advantages of C# over Python