The compelling reason for me is F#'s compiler is far smarter, catching entire types of bugs that the C# compiler will not. For example, if you use option types, you can almost entirely eliminate null reference exceptions, as the compiler will complain about misuse of an option. The lack of default circular references on types enforces a real layered architecture, so you can open up any F# project and read it top to bottom, with the lowest layers coming first. This post has a great explanation of that feature: http://fsharpforfunandprofit.com/posts/cyclic-dependencies/

Also, with default immutability, type providers, and less boilerplate I feel a lot more confident with the quality of the code I write, and I am able to write it faster.

Since it's possible to do full C#-style OO in F#, I personally don't see a reason to use C# at all. F# is more supporting of a multi paradigm codebase, it's safer, and requires less code to do the same things as C#. Combine that with what I would consider to be an extremely short learning curve, I am happy finding C# programmers and letting them learn F# the first week of the job.