Both languages have a rich development ecosystem, but I would vote for .NET on the basis that C#, the most common .NET language is statically typed. This is a significant productivity boost for developers, as the integrity of your program is checked at compile-time as opposed to run-time.
Source: http://programmers.stackexchange.com/questions/10032/dynamic...
You can also theoretically reuse any existing Ruby libraries with IronRuby but I don't know how good that is because I've never done it.
There are licensing costs with .NET but these are usually not significant if the company is well-established. Considering your company can apparently afford two teams that would appear to be the case.
Note: I have been a .NET developer for the last 6 years, and while I have used Ruby on Rails and Chef a fair bit, I may have missed some important features of Ruby. I'd love to know if there are any killer features that I have overlooked.