In a recent analysis of venture capital returns for companies based on programming language, for the top 50 Y Combinator companies at least as of 2019, Ruby is #1, and C# is just not in the list.

https://charliereese.ca/y-combinator-top-50-software-startup...

I have no affiliation with Y Combinator itself or the author of the post.

The particular analysis is pretty skewed in its own way, so you can take it with a pinch of salt.

As a general rule, programming languages have ecosystems and communities, and if you pick a language where quickly delivering high-value features to users/customers is part of the community, it helps your startup; you'll probably meet founders and get business advice if you go to a Ruby meetup, but less so at a C# one.

It's not the whole picture, since it doesn't speak to the technical part of your question.