Another thing to keep in mind is the political aspect of choosing a tech stack, especially for a startup... As you'll surely notice right here in the comments, MS tools are not what you could call fashionable in many tech circles, and that could hamper a startup ability to raise money, or affect it's market value unless it's a smash hit like Stack Overflow.

It's up for debate whether the bad reputation is deserved by Microsoft in 2013 or not, but fairness is not what's important here...

For my part I once used C# in the past for a couple of projects and I have very fond memories of it. It's a well balanced language with very solid tooling, and the Mono stack seems nice as well. In a way, I'd be happy to use it again on a new project... but at the same time I must admit that being rid of the stigma of using MS tech is a pleasant feeling. And it's indeed expensive, although it has never been my job to worry about that.

In a way, for me, MS tools are like that old out-of-fashion shirt that you I still like to wear from time to time even though you know people will think it looks weird.

To my wife, if she reads this: Sorry, I'm not getting rid of the shirt.