What is the C# role doing? Is it moving your career/knowledge forward in any way (other than learning C#)?

It just seems like an odd choice (C#) given what you want to do. Scientific computing is C++, Fortran, and Python, along with some Matlab, R, maybe Julia. A ton of machine learning is in R and Python. Of course Java is heavily used in data analytics due to Hadoop, so C## will help you there.

But really, it's pretty trivial to learn a language. C# is a fantastic language, but it seems that it is likely that your role will be taking you further away from your future plans, not nearer them.

I don't feel super strongly about either choice, other than pick the work that is most interesting, rather than pick based on language. I'm posting mostly because all other replies are pro C# at the moment, and I wanted to offer a contrast.