IMO it just depends on the servers you're going to be using and who you'll be working with. I would stay away, far away from traditional ASP.NET. MVC is far better, but I would only really use it over Rails if I was working on a primarily MS stack environment.
Do you think retail banking sites, trading firms, brokerages, etc. would choose .NET today?
If that all seems like it trends corporate for you, well, that's obviously going to happen when your platform costs money.
All else being equal, would I start a new project on Microsoft's stack? No. But if I was really strong on ASP.NET and completely new to Django or Rails, I'd stick with ASP.NET. It works, it works at scale, and it's far from the hardest stack to code for. I might even suggest it's better than PHP, all else being equal.