Python/Django or C#/ASP.NET for web development?
Derek had a great answer, so I won't repeat it. 
I would like to make one observation, however. While for the most part, the language choice isn't really a big deal these days, if you really need high performance and scalability, the dynamic nature of python might come back to haunt you. For all the benefits that a dynamic language can provide, those benefits do come at the cost of additional overhead. The .NET platform offers fully compiled languages, and offers a variety of ways to tune the performance of compiled code (including ngen support, so you can create natively compiled modules that do not need to be JITted.) 
I do not know if the performance edge .NET compiled languages have over Python is really enough for your particular application, but given that you are already a .NET developer, going with ASP.NET might be the best option.
I am a .NET developer. I have no experience on Python. Which platform is more scalable? Which platform is more suitable for large-size high-traffic web sites? If you have any experience about scalability on these platforms, please inform us.
Thank you.