I've found that as C# gains much of the features that F# has and will soon gain more (pattern matching, functions as first class data types, great fp libraries, etc) the "moat" that F# has over C# has gotten smaller. I write most of my c# code in a primarily functional style, but I still have the advantage of using the libraries in their own native ways that follow the examples given by microsoft and other vendors.
Watching c# eat f# features as someone who has dabbled in f# lightly for over a decade has been wild. And supposedly DU's are in the works but multiple years out.
Though one thing I doubt c# ever gets that I love when I'm writing f# is pipeline operators. I love the way they read, from object/collection being worked on and then a list of operations being run on it in order from left to right (or you can do right to left if you need to for some particular reason).
