If you "rarely have performance problems in reality" that explains a lot. Speed differences between Ruby and NET (see the link in my previous post) translate to having one server instead of twenty. If you make low traffic solutions, that's OK. If you move bottlenecks to something outside of your responsibility (like for example, your app waits on sql server because no caching is done) OK, not your problem. If somebody doesn't have problems in buying more hardware, OK.
Still it's approx. 20-times (see the link before, it is for Ruby 1.9) trade-off between development convenience and the hardware involved.
If you really need a rack full of Ruby servers to do the job of a single .NET server, your Ruby programmers are doing something (hint: most probably everything) wrong. Language choice excepted.
Server performance is only very rarely tied to CPU performance. There are a whole lot of factors other than that that impact performance.
Still it's approx. 20-times (see the link before, it is for Ruby 1.9) trade-off between development convenience and the hardware involved.