Didn't you say "next generation"? Also I'm not sure if Django is the perfect example for a SPA. Sure it can return json (exactly as plain
asp.net mvc can do very well), but it's not ideal for many reasons. Have a look at
http://stackoverflow.com/questions/713847/recommendations-of-python-rest-web-services-framework and check out microframeworks like flask, maybe tornado..
Decision between python and C# is really important, and the question about how many projects are running on servicestack is probably the least important. What is the current platform at your company, is the team more experienced in C# or python, what about the operations team? Are you planning to have multiple instances (means license costs unless your boss feels a little bit adventurous and wants to try mono, which I doubt judging from the question). I'm sure you already asked those questions, but I can't imagine that the pros/cons would be so even.
Back to the question, I would love to know the answer too. The website is a bit shy on references/customers. I know several projects that are very happily using some parts of the servicestack family (redis client, ormlite, json serializer), but I haven't personally seen anyone using the full stack.
Thanks
Tom