Unifying technology eliminates or reduces that risk.
If the goal is to determine which delivers more value, then the place to start is by measuring value...and that means deciding what constitutes value and building systems to measure it. But really, this seems like a cultural decision and while cultural decisions can create and destroy value within a company they don't deliver it to customers. Even worse, they are hard and don't have a right answer and if you get it wrong good people leave and if you get it really wrong it's hard to hire good people to replace them. Even getting it right means that good people might leave.
And people might not leave for the first order reasons. A Rubyist might leave because a .NET developer left because the choice was Ruby and that didn't sit right with the Ruby developer.
Anyway, if people going on holiday is a staffing problem then the problem is not enough staff or adequate management planning. It is not Ruby versus .NET: it is a management problem.
Good luck.
I'm a Ruby guy, but I understand .NET has taken some leaps and bounds in the last 5 years, especially since Microsoft ported it to Linux. There's no clear winner yet for me personally. YMMV.