Regarding the language doesn't matter thing I think that's mostly true. If you're choosing between Ruby and ASP.Net MVC I don't think you can loose. But sometimes the framework does matter, like if you want to build a startup on webforms (don't).
I think you're right about WebForms in the sense that I can't think of a single good reason to go with it over MVC. I maintain that it was never really intended for the web we use these days anyway.
It was designed to make web development "simple" for the kind of people who built WinForms applications and were too lazy to learn how the web actually works.
Thankfully Microsoft have learnt from their errors so are now pushing these types towards making a mess of Javascript. Visual Basic types will be happy to learn that semicolons are optional.
If you have to, you can change later- though from experience, there's little need to with .NET. I work for RecordSetter (http://www.recordsetter.com) and we use ASP.NET MVC- back in the early days of the company, the CTO knew .NET very well, so he just used his existing knowledge. C# is a great language, Visual Studio is a great IDE, MVC is a great framework. And so on and so on. We aren't really suffering any ill effects as a result of our choice, and as others have noted, BizSpark membership is actually a great bonus.
If anyone out there wants to play around with C# for the web but feels like the ASP.NET stack is too heavy (it can be), take a look at Nancy.FX (http://www.nancyfx.org/). It's super lightweight and allows you to keep most of .NET out of your way.