F# for several reasons 1) Tooling, I like proper thick IDE's like xamarins or VS. 2) better platform support (I'm serious, it isnt tied so much to windows as you might think but is one of few languages you can e.g make a 3-platform mobile app with today!). Running Haskell in the same IDE with the same compiler on several platforms is hard. Making android/iOS or web apps with it is also tricky. 3) I like ML more than Haskell, 4) trivial to slap a non-functional C# front end on F# business logic should that be necessary, much harder to do from Haskell or ML without having to resort to C/C++.
Xamarin is really the "killer app" for .NET at the moment. Being able to use a common codebase for Windows RT, Windows Phone, Windows, OS-X, Android, iOS is amazing. Being able to do full native work on top of that on every platform. The apps look, feel and act native cause they are using native widgets.
Last I checked the F# supports was still a little crufty (over a year ago) -- but if they really get the F# support to be top tier, I will have to return and have another look.
F# has gone from tiny to quite popular (#12 spot on Tiobe currently), and that shows very clearly in the quality of tools and availability of libs now. It's not the same language/community it was a year ago.
By comparison, Haskell is 49 and no ML lang is in the top 50.
I feel like the near-reliance on Windows is the biggest issue for F#. Haskell fits the Unix environment a lot better.