It is easy to say do another one, but without funding/investors this can be hard and people must eat and bills must be paid.
Hence with .NET being the choice of most businesses (as most they are already tooled with MS products etc), going .NET initially also allows the fallback position of easier consulting/contract work at times when money is needed.
However, ultimately the choice should be on what allows you to meet and iterate upon your objectives in the fastest way possible, as the best way to stay live is to be able to adapt and change and exploit markets rapidly. Hence with this in mind, I would opt for Ruby as imho it provides a better grounding into architectures and pratices that will best serve you in both the short and longer terms.
Hope this is of help :)
We looked for Ruby devs, quickly gave up, and wound up hiring a Lisp hacker. Our runner-up was a sharp C# hacker.
There is a huge amount to programming .net that has nothing to do with the {}'s of C# and all to do with the .net framework (which won't translate to Ruby at all).
That said, a good/sharp programmer is more than a monkey slinging code so they should be able to work well in any language.
The reality is, learning framework features is no different than learning how to mail-merge with your word processor. Sure, it's important. But it has nothing to do with whether someone's going to do a good job coding for you, 2 months in.